Add foundations for CMake support

.gitignore

@@ -1 +1,2 @@
 /bazel-*
+cmake/build*

CMakeLists.txt

@@ -0,0 +1,30 @@
+# Copyright 2024 Aurora Operations, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# CMake should always be at least as new as your compiler[1].  Clang 14 was released on March 25,
+# 2022, and is our newest officially supported compiler.  The next CMake release was v3.23.0, on
+# March 29, 2022.  Therefore, our minimum version must be at least CMake 3.23.
+#
+# The maximum version should be the latest version we've tested the project with.
+#
+# [1]: https://cliutils.gitlab.io/modern-cmake/chapters/intro/dodonot.html
+cmake_minimum_required(VERSION 3.23...3.29)
+
+project(
+   Au
+   VERSION 0.3.4
+   DESCRIPTION "A C++ quantities and units library, by Aurora Innovation"
+   HOMEPAGE_URL "https://aurora-opensource.github.io/au/0.3.4/"
+   LANGUAGES CXX
+)

RELEASE.md

@@ -52,6 +52,17 @@ Any empty section can be omitted.
 We try to follow [semantic versioning](https://semver.org/).  Since we are currently in major
 version zero (0.y.z), incompatible changes don't force a major version upgrade.
 
+### Update the CMake version number
+
+Edit the `CMakeLists.txt` file in the root folder, updating the version number in the `project`
+command to the number chosen above.
+
+Also update the version number in the `HOMEPAGE_URL` parameter, because we link to the docs for the
+latest release in our CMake project definition.  (True, this URL won't exist until you complete the
+remaining steps in this guide, but the danger of getting it wrong is pretty small.)
+
+Make a PR with these changes and land it before creating the tag.
+
 ### Fill out release notes template
 
 The first line should be the tag name.
@@ -102,7 +113,9 @@ Issues!  Alphabetically:
 
 ### Create the tag
 
-Use the command below, replacing `0.3.1` with the version to create.
+First, make sure the "final commit" (which updates the CMake variables) has already landed.
+
+Then, use the command below, replacing `0.3.1` with the version to create.
 
 ```sh
 # Remember to update the tag number!

docs/install.md

@@ -247,6 +247,22 @@ attribute, and include the appropriate files.
 | `@au//au:io` | `"au/io.hh"` | `operator<<` support |
 | `@au//au:testing` | `"au/testing.hh"` | Utilities for testing<br>_Note:_ `testonly = True` |
 
+#### CMake
+
+CMake support is still experimental and in-progress.  We are building it up starting from "root"
+targets (that is, targets without dependencies), and expanding support to the rest of the library.
+
+To build the library using this experimental CMake support, follow these steps:
+
+```sh
+# CMake is a "meta build system", not a build system.
+# This first command generates the actual build files.
+cmake -B cmake/build -S .
+
+# This command builds the library.
+cmake --build cmake/build
+```
+
 #### Other build systems (CMake / conan / vcpkg / ...)
 
 We would like to support all these build and packaging systems, and perhaps others!  But the initial