Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (0da8145f3e741ea79ab540ce47df092139741151)


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

README.md

@@ -51,3 +51,5 @@ load_model = load_from_hub(
 model = A2C.load(load_model)
 
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)