add panda gym env paper

README.md

@@ -83,4 +83,4 @@ OrderedDict([('batch_size', 256),
 {'render_mode': 'rgb_array'}
 ```
 
-Panda Gym environments: arxiv.org/abs/2106.13687
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)