Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (a34b3941220447eba71f62344a12abe31c19f662)


Co-authored-by: Quentin Gallouédec <qgallouedec@users.noreply.huggingface.co>

README.md

@@ -34,3 +34,5 @@ from huggingface_sb3 import load_from_hub
 
 ...
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)