docs: update README

Author: ChenZiHong-Gavin

README.md

@@ -8,6 +8,7 @@
 [![forks](https://img.shields.io/github/forks/open-sciencelab/GraphGen.svg)](https://github.com/open-sciencelab/GraphGen)
 [![open issues](https://img.shields.io/github/issues-raw/open-sciencelab/GraphGen)](https://github.com/open-sciencelab/GraphGen/issues)
 [![issue resolution](https://img.shields.io/github/issues-closed-raw/open-sciencelab/GraphGen)](https://github.com/open-sciencelab/GraphGen/issues)
+[![documentation](https://img.shields.io/badge/docs-latest-blue)](https://graphgen-cookbook.readthedocs.io/en/latest/)
 
 GraphGen: Enhancing Supervised Fine-Tuning for LLMs with Knowledge-Driven Synthetic Data Generation