mention pybullet in readme

README.rst

@@ -94,7 +94,7 @@ On Ubuntu 16.04 and 18.04:
 MuJoCo has a proprietary dependency we can't set up for you. Follow
 the
 `instructions <https://github.com/openai/mujoco-py#obtaining-the-binaries-and-license-key>`_
-in the ``mujoco-py`` package for help.
+in the ``mujoco-py`` package for help.  As an alternative to ``mujoco-py``, consider `PyBullet <https://github.com/openai/gym/blob/master/docs/environments.md#pybullet-robotics-environments>`_ which uses the open source Bullet physics engine and has no license requirement.
 
 Once you're ready to install everything, run ``pip install -e '.[all]'`` (or ``pip install 'gym[all]'``).
 
@@ -130,7 +130,7 @@ maintain the lists of dependencies on a per-environment group basis.
 Environments
 ============
 
-See `List of Environments <docs/environments.md>`_.
+See `List of Environments <docs/environments.md>`_ and the `gym site <http://gym.openai.com/envs/>`_.
 
 For information on creating your own environments, see `Creating your own Environments <docs/creating-environments.md>`_.