Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (3b019bf93bc1893d15fce122b760a3a17daa0ad1)


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

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)