Update documentation for OpenMPI (#437)

* Update documentation

* Fix hyperlink in changelog

* Restore OpenMPI label

* Update README.md

README.md

@@ -85,9 +85,12 @@ brew install cmake openmpi
 To install stable-baselines on Windows, please look at the [documentation](https://stable-baselines.readthedocs.io/en/master/guide/install.html#prerequisites).
 
 ### Install using pip
-Install the Stable Baselines package
+Install the Stable Baselines package:
+```
+pip install stable-baselines[mpi]
+```
 
-Using pip from pypi:
+This includes an optional dependency on MPI, enabling algorithms DDPG, GAIL, PPO1 and TRPO. If you do not need these algorithms, you can install without MPI:
 ```
 pip install stable-baselines
 ```

docs/guide/install.rst

@@ -47,28 +47,27 @@ We recommend using `Anaconda <https://conda.io/docs/user-guide/install/windows.h
 (using again ``pip install -e .``)
 
 
+.. _openmpi:
+
 Stable Release
---------------
+~~~~~~~~~~~~~~
+To install with support for all algorithms, including those depending on OpenMPI, execute:
 
 .. code-block:: bash
 
-    pip install stable-baselines
-
-
-.. _openmpi:
+    pip install stable-baselines[mpi]
 
-Stable Release with OpenMPI
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 GAIL, DDPG, TRPO, and PPO1 parallelize training using OpenMPI. OpenMPI has had weird
 interactions with Tensorflow in the past (see
-`Issue #430 <https://github.com/hill-a/stable-baselines/issues/430>`) and so is disabled by
-default.
+`Issue #430 <https://github.com/hill-a/stable-baselines/issues/430>`_) and so if you do not
+intend to use these algorithms we recommend installing without OpenMPI. To do this, execute:
 
 .. code-block:: bash
 
-    pip install stable-baselines[mpi]
+    pip install stable-baselines
 
-To disable OpenMPI, uninstall ``mpi4py`` with ``pip uninstall mpi4py``.
+If you have already installed with MPI support, you can disable MPI by uninstalling ``mpi4py``
+with ``pip uninstall mpi4py``.
 
 
 Bleeding-edge version