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

@@ -67,3 +67,5 @@ OrderedDict([('batch_size', 256),
               'n_sampled_goal=4 )'),
              ('normalize_kwargs', {'norm_obs': True, 'norm_reward': False})])
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)