Add documentation around plugins and formatters

docs/source/dev/formatters.rst

@@ -0,0 +1,16 @@
+===========================================
+ Developing a Formatting Plugin for Flake8
+===========================================
+
+Flake8 added the ability to develop custom formatting plugins in version
+3.0.0. Let's write a plugin together:
+
+.. code-block:: python
+
+    from flake8.formatting import base
+
+
+    class Example(base.BaseFormatter):
+        """Flake8's example formatter."""
+
+        pass

docs/source/dev/registering_plugins.rst

@@ -0,0 +1,113 @@
+==================================
+ Registering a Plugin with Flake8
+==================================
+
+To register any kind of plugin with Flake8, you need a few things:
+
+#. You need a way to install the plugin (whether it is packaged on its own or
+   as part of something else). In this section, we will use a ``setup.py``
+   written for an example plugin.
+
+#. A name for your plugin that will (ideally) be unique.
+
+#. A somewhat recent version of setuptools (newer than 0.7.0 but preferably as
+   recent as you can attain).
+
+Flake8 presently relies on a functionality provided by setuptools called
+`Entry Points`_. These allow any package to register a plugin with Flake8 via
+that package's ``setup.py`` file.
+
+Let's presume that we already have our plugin written and it's in a module
+called ``flake8_example``. We might have a ``setup.py`` that looks something
+like:
+
+.. code-block:: python
+
+    from __future__ import with_statement
+    import setuptools
+
+    requires = [
+        "flake8 > 3.0.0",
+    ]
+
+    flake8_entry_point = # ...
+
+    setuptools.setup(
+        name="flake8_example",
+        license="MIT",
+        version="0.1.0",
+        description="our extension to flake8",
+        author="Me",
+        author_email="example@example.com",
+        url="https://gitlab.com/me/flake8_example",
+        packages=[
+            "flake8_example",
+        ],
+        install_requires=requires,
+        entry_points={
+            flake8_entry_point: [
+                'X = flake8_example.ExamplePlugin',
+            ],
+        },
+        classifiers=[
+            "Environment :: Console",
+            "Intended Audience :: Developers",
+            "License :: OSI Approved :: MIT License",
+            "Programming Language :: Python",
+            "Programming Language :: Python :: 2",
+            "Programming Language :: Python :: 3",
+            "Topic :: Software Development :: Libraries :: Python Modules",
+            "Topic :: Software Development :: Quality Assurance",
+        ],
+    )
+
+Note specifically these lines:
+
+.. code-block:: python
+
+    flake8_entry_point = # ...
+
+    setuptools.setup(
+        # snip ...
+        entry_points={
+            flake8_entry_point: [
+                'X = flake8_example.ExamplePlugin',
+            ],
+        },
+        # snip ...
+    )
+
+We tell setuptools to register our entry point "X" inside the specific 
+grouping of entry-points that flake8 should look in.
+
+Flake8 presently looks at three groups:
+
+- ``flake8.extension``
+
+- ``flake8.format``
+
+- ``flake8.report``
+
+If your plugin is one that adds checks to Flake8, you will use
+``flake8.extension``. If your plugin formats the output and provides that to
+the user, you will use ``flake8.format``. Finally, if your plugin performs
+extra report handling (filtering, etc.) it will use ``flake8.report``.
+
+If our ``ExamplePlugin`` is something that adds checks, our code would look
+like:
+
+.. code-block:: python
+
+    setuptools.setup(
+        # snip ...
+        entry_points={
+            'flake8.extension': [
+                'X = flake8_example.ExamplePlugin',
+            ],
+        },
+        # snip ...
+    )
+
+
+.. _Entry Points:
+    https://pythonhosted.org/setuptools/pkg_resources.html#entry-points