Add link to Panda Gym paper (#2)

- Add link to Panda Gym paper (f6a9573b5c5cf0e6bc31c55f01d4bb4a458266a9)

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)