add paper source

add panda-gym paper

README.md

@@ -83,3 +83,5 @@ OrderedDict([('batch_size', 2048),
 ```python
 {'render_mode': 'rgb_array'}
 ```
+
+Panda Gym environments: [arxiv.org/abs/2106.13687](https://arxiv.org/abs/2106.13687)