Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (752f29cd6a41099a2cb1a3562f1f4d9da2c062cb)


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)