Merge #302: rewrite README.md

c8602cf rewrite README.md (Andrew Poelstra)

Pull request description:

  With suggestions and rewording from Claude 4.
  
  The next PR will attempt to fill in the top-level library docs which appear on docs.rs, including more useful simple example programs.


Top commit has no ACKs.


Tree-SHA512: cfaeba995028264dd23fdbdba68e72979ab3854c355f487d8596caa73cb23f795703b1acb08ce5ec4d5b0922d0cc27cf6b4f14bb71c01d2c162650b022f58270

Author: apoelstra

README.md

@@ -1,19 +1,137 @@
-![Build](https://github.com/apoelstra/rust-simplicity/workflows/Continuous%20integration/badge.svg)
+<p>
+  <a href="https://crates.io/crates/simplicity-lang"><img alt="Crate Info" src="https://img.shields.io/crates/v/simplicity-lang.svg"/></a>
+  <a href="https://github.com/BlockstreamResearch/rust-simplicity/blob/master/LICENSE"><img alt="CC0 1.0 Universal Licensed" src="https://img.shields.io/badge/license-CC0--1.0-blue.svg"/></a>
+  <a href="https://github.com/BlockstreamResearch/rust-simplicity/actions?query=workflow%3AContinuous%20integration"><img alt="CI Status" src="https://img.shields.io/github/actions/workflow/status/BlockstreamResearch/rust-simplicity/main.yml"></a>
+  <a href="https://docs.rs/simplicity-lang"><img alt="API Docs" src="https://img.shields.io/badge/docs.rs-simplicity_lang-green"/></a>
+</p>
 
 # rust-simplicity
-Under development....
 
-# Minimum Supported Rust Version
+This is the official Rust library of the [Simplicity Language](https://simplicity-lang.org/).
 
-The MSRV of this crate is **1.78.0**.
+Simplicity is a low-level, typed functional language designed to be a drop-in alternative
+for Bitcoin's [Tapscript](https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki).
+It offers static resource bounds and has a formal specification (in Rocq) which allows the
+creation of machine-checkable proofs of program behavior.
 
-# Updating jets code
+It is currently deployed on Blockstream's Liquid, which is a sidechain resembling Bitcoin
+in many ways; but which differs in many Script-relevant ways (e.g. supporting multiple
+assets and using Confidential Transactions). We expect by the end of 2025 to directly
+support Bitcoin, so that Simplicity can be used on a custom regtest chain.
 
-Some of the jet files in the library are auto-generated from Haskell code. These can be updated `update_jets.sh`. This requires the user has `cabal` and other necessary things that are required to build simplicity haskell. Instructions for those can be found in the simplicity repository.
+Simplicity is a very low-level language. If you are simply looking to develop with the
+language, you may wish to use [SimplicityHL](https://github.com/BlockstreamResearch/simplicityhl)
+instead.
 
-This script also checks that the internal vendored version of simplicity has the same git hash as of the version from which we are auto-generating the code. If this is not the case, the script will fail. This is because we only vendor minimal required C simplicity code and not the entire simplicity repo.
+## rust-simplicity
 
-# Benchmarking
+The goal of rust-simplicity is to provide the tools to encode, decode, analyze, construct
+and manipulate Simplicity programs at all stages of development. These stages are:
+
+* **construction**, which is when a program is incomplete, and consists of a set of
+  "Simplicity expressions" intended to be assembled into a full program. Such expressions
+  are represented by the [`ConstructNode`](https://docs.rs/simplicity-lang/latest/simplicity/node/type.ConstructNode.html)
+  type.
+* **commitment**, which is when a program is complete but has not been satisfied. At this
+  point, it is possible to derive addressses and receive funds to them, but to spend
+  those funds requires the generation of witness data. Such programs are represented
+  by [`CommitNode`](https://docs.rs/simplicity-lang/latest/simplicity/node/type.CommitNode.html)
+* **redemption**, which is when a program has witness data attached. This is the only
+  point at which a program may appear explicitly on the blockchain, and the only point
+  at which the program has a canonical encoding. Such programs are represented by
+  [`RedeemNode`](https://docs.rs/simplicity-lang/latest/simplicity/node/type.RedeemNode.html)
+
+Generally speaking, the process of producing a Simplicity program involves moving
+from one node type to the next.
+
+## Installation
+
+To use rust-simplicity, this to your Cargo.toml:
+
+```toml
+[dependencies]
+simplicity-lang = "0.5"
+```
+
+## Quick Start
+
+```rust
+use simplicity::node::CoreConstructible;
+use simplicity::types::Context;
+use simplicity::{ConstructNode, Core};
+use std::sync::Arc;
+
+// Create a trivial Simplicity program
+let ctx = Context::new();
+let program = Arc::<ConstructNode<Core>>::unit(&ctx);
+
+// Encode the program to bytes
+let encoded = simplicity::bit_encoding::write_to_vec(|w| {
+    program.encode_without_witness(w)
+}).unwrap();
+```
+
+## Relationship to libsimplicity
+
+This Rust library provides extensive tools for program construction and analysis, while
+[libsimplicity](https://github.com/BlockstreamResearch/Simplicity) is a minimal C library
+focused solely on decoding, validating, and executing programs in blockchain consensus code.
+
+rust-simplicity includes a vendored copy of libsimplicity and uses it for:
+
+- cryptographic and other "heavy" operations through optimized C jets
+- cross-validating the C and Rust Bit Machine implementations
+
+The libsimplicity repository also contains:
+- A reference implementation written in Haskell
+- A Rocq (Coq) implementation for formal verification and machine-checkable proofs
+
+## Further Reading
+
+- **[API Documentation](https://docs.rs/simplicity-lang/)** - Complete API reference
+- **[Simplicity Tech Report](https://github.com/BlockstreamResearch/simplicity/blob/pdf/Simplicity-TR.pdf)** - Full informal language specification
+- **[Simplicity Website](https://simplicity-lang.org/)** - Official language homepage
+- **[libsimplicity](https://github.com/BlockstreamResearch/Simplicity)** - Reference implementation
+
+# Contributing
+
+The goals of this library are to support the low-level Simplicity language, and to
+provide a basis for the [SimplicityHL](https://github.com/BlockstreamResearch/simplicityhl)
+language implementation. If you wish to work on high-level programming language
+abstractions, you may want to visit that repo instead.
+
+Having said that, contributions are welcome! This library is in its early stages
+and undergoing rapid development. We especially welcome improvements in:
+
+* representing and displaying type inference information and errors
+* representing and displaying information about Simplicity (sub)expressions;
+  usefully and uniquely specifying subexpressions within larger expressions
+* improvements to the debuggability and introspection of the Bit Machine, the
+  Simplicity interpreter.
+* testing, especially of programs that exhibit extreme behavior; improving test
+  infrastructure and CI
+* improving the ergonomics and reliability of our vendoring scripts (see below)
+* documentation!
+
+## Minimum Supported Rust Version
+
+The MSRV of this crate is **1.78.0**. For now we will not increase this without
+a major version bump, though as the library matures we may change that policy.
+
+## Updating libsimplicity
+
+This library includes a vendored copy of the libsimplicity C library, as well as
+files generated by the `GenRustJets` utility from the libsimplicity Haskell library.
+These files are clearly marked as being autogenerated, and may not be directly
+changed.
+
+Instead, to update this code you must use the included `vendor-simplicity.sh` and
+`update_jets.sh` scripts.
+
+`update_jets.sh` requires the user has `cabal` and all dependencies of the libsimplicity
+Haskell library. Instructions for those can be found in the simplicity repository.
+
+## Benchmarking
 
 There are two sets of benchmarks in this codebase. First, there is the `jets-bench`
 sub-crate which uses criterion to collect statistics about jet performance. These