Add installation instructions to README (#148)

Added installation instructions for building from source.

Author: mergennachin

README.md

@@ -1,6 +1,13 @@
 # tokenizers
 C++ implementations for various tokenizers (sentencepiece, tiktoken etc). Useful for other PyTorch repos such as torchchat, ExecuTorch to build LLM runners using ExecuTorch stack or AOT Inductor stack.
 
+## Installation (from source)
+```
+git clone [email protected]:meta-pytorch/tokenizers.git
+cd ~/tokenizers
+git submodule update --init --recursive
+pip install -e .
+```
 
 ## SentencePiece tokenizer
 Depend on https://github.com/google/sentencepiece from Google.