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

@@ -62,4 +62,5 @@ model.learn(1_000_000, progress_bar=True, callback=WandbCallback(verbose=2))
 wandb_run.finish()
 ```
 
-Weights & Biases charts: https://wandb.ai/patonw/PandaPickAndPlace-v3/runs/w7lzlwnx/workspace?workspace=user-patonw
+Weights & Biases charts: https://wandb.ai/patonw/PandaPickAndPlace-v3/runs/w7lzlwnx/workspace?workspace=user-patonw
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)