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

@@ -109,3 +109,5 @@ mean_reward, std_reward = evaluate_policy(model, eval_env)
 print(f"Mean reward = {mean_reward:.2f} +/- {std_reward:.2f}")
 ...
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)