Rename dev subdirectory to plugin-development

This should make the contents clearer

docs/source/dev/registering_plugins.rst

@@ -1,115 +0,0 @@
-.. _register-a-plugin:
-
-==================================
- Registering a Plugin with Flake8
-==================================
-
-To register any kind of plugin with |Flake8|, 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| relies on 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.listen``
-
-- ``flake8.report``
-
-If your plugin is one that adds checks to |Flake8|, you will use
-``flake8.extension``. If your plugin automatically fixes errors in code, you
-will use ``flake8.listen``.  Finally, if your plugin performs extra report
-handling (formatting, 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