Add pipeline tag, library name and link to paper and Github repository

This PR improves the model card by:

- Adding the `pipeline_tag`, ensuring people can find the model under https://huggingface.co/models?pipeline_tag=graph-ml
- Adding `library_name: transformers`
- Adding a link to the paper
- Adding a link to the Github repository

README.md

@@ -1,3 +1,9 @@
----
-license: apache-2.0
----
+---
+license: apache-2.0
+pipeline_tag: graph-ml
+library_name: transformers
+---
+
+This is the G1 model based on Qwen, presented in [G1: Teaching LLMs to Reason on Graphs with Reinforcement Learning](https://huggingface.co/papers/2505.18499).
+
+For more information and code, please see the Github repository: https://github.com/PKU-ML/G1