Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (3a39c1c960f8e33855d8fd3f67273cf1684b7d4c)


Co-authored-by: Quentin Gallouédec <[email protected]>

README.md

@@ -35,3 +35,5 @@ from huggingface_sb3 import load_from_hub
 
 ...
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)