Add link to Panda Gym paper (#1)

- Add link to Panda Gym paper (8e1c624a3c6b189a652c9f2f16c1ed9554412334)


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

README.md

@@ -26,3 +26,5 @@ This is a trained model of a **A2C** agent playing **PandaReachDense-v2**
 using the [stable-baselines3 library](https://github.com/DLR-RM/stable-baselines3).
 
 
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)