Add link to Panda Gym paper

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)