Add some notes for users about how to use plugins

docs/source/user/index.rst

@@ -24,6 +24,7 @@ This guide will cover all of these and the nuances for using |Flake8|.
     configuration
     options
     ignoring-errors
+    using-plugins
     python-api
 
 .. config files

docs/source/user/using-plugins.rst

@@ -0,0 +1,66 @@
+==================================
+ Using Plugins For Fun and Profit
+==================================
+
+|Flake8| is useful on its own but a lot of |Flake8|'s popularity is due to
+its extensibility. Our community has developed :term:`plugin`\ s that augment
+|Flake8|'s behaviour. Most of these plugins are uploaded to PyPI_. The
+developers of these plugins often have some style they wish to enforce.
+
+For example, `flake8-docstrings`_ adds a check for :pep:`257` style
+conformance. Others attempt to enforce consistency, like `flake8-future`_.
+
+.. note::
+
+    The accuracy or reliability of these plugins may vary wildly from plugin
+    to plugin and not all plugins are guaranteed to work with |Flake8| 3.0.
+
+To install a third-party plugin, make sure that you know which version of
+Python (or pip) you used to install |Flake8|. You can then use the most
+appropriate of:
+
+.. prompt:: bash
+
+    pip install <plugin-name>
+    pip3 install <plugin-name>
+    python -m pip install <plugin-name>
+    python2.7 -m pip install <plugin-name>
+    python3 -m pip install <plugin-name>
+    python3.4 -m pip install <plugin-name>
+    python3.5 -m pip install <plugin-name>
+
+To install the plugin, where ``<plugin-name>`` is the package name on PyPI_.
+To verify installation use:
+
+.. prompt:: bash
+
+    flake8 --version
+    python<version> -m flake8 --version
+
+To see the plugin's name and version in the output.
+
+.. seealso:: :ref:`How to Invoke Flake8 <invocation>`
+
+After installation, most plugins immediately start reporting :term:`error`\ s.
+Check the plugin's documentation for which error codes it returns and if it
+disables any by default.
+
+.. note::
+
+    You can use both :option:`flake8 --select` and :option:`flake8 --ignore`
+    with plugins.
+
+Some plugins register new options, so be sure to check :option:`flake8 --help`
+for new flags and documentation. These plugins may also allow these flags to
+be specified in your configuration file. Hopefully, the plugin authors have
+documented this for you.
+
+.. seealso:: :ref:`Configuring Flake8 <configuration>`
+
+
+.. _PyPI:
+    https://pypi.io/
+.. _flake8-docstrings:
+    https://pypi.io/project/flake8-docstrings/
+.. _flake8-future:
+    https://pypi.io/project/flake8-future/