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

@@ -15,3 +15,5 @@ Sample pictures of this concept:
   ![0](https://huggingface.co/NancyAthghara23/red-panda-rpd/resolve/main/sample_images/00004-3404897571.png)
       ![1](https://huggingface.co/NancyAthghara23/red-panda-rpd/resolve/main/sample_images/00006-1635300479.png)
       
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)