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

@@ -41,4 +41,5 @@ Report any issues with the demo at the [github repo](https://github.com/nateraw/
 
 #### wombat
 
-![wombat](images/wombat.jpg)
+![wombat](images/wombat.jpg)
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)