Add link to Panda Gym paper

Hi there,
In this PR, I invite you to add a link to Panda Gym in the description of your model, it helps to better centralize all the models that use its environments. Take a look at https://huggingface.co/papers/2106.13687. This PR was generated autamically, so forgive me if it's not relevant.
Thanks for your time!

README.md

@@ -25,3 +25,5 @@ model-index:
 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)