Rename dev subdirectory to plugin-development

This should make the contents clearer

docs/source/dev/index.rst

@@ -1,56 +0,0 @@
-============================
- Writing Plugins for Flake8
-============================
-
-Since |Flake8| 2.0, the |Flake8| tool has allowed for extensions and custom
-plugins. In |Flake8| 3.0, we're expanding that ability to customize and
-extend **and** we're attempting to thoroughly document it. Some of the
-documentation in this section may reference third-party documentation to
-reduce duplication and to point you, the developer, towards the authoritative
-documentation for those pieces.
-
-Getting Started
-===============
-
-To get started writing a |Flake8| :term:`plugin` you first need:
-
-- An idea for a plugin
-
-- An available package name on PyPI
-
-- One or more versions of Python installed
-
-- A text editor or IDE of some kind
-
-- An idea of what *kind* of plugin you want to build:
-
-  * Formatter
-
-  * Check
-
-Once you've gathered these things, you can get started.
-
-All plugins for |Flake8| must be registered via `entry points`_. In this
-section we cover:
-
-- How to register your plugin so |Flake8| can find it
-
-- How to make |Flake8| provide your check plugin with information (via
-  command-line flags, function/class parameters, etc.)
-
-- How to make a formatter plugin
-
-- How to write your check plugin so that it works with |Flake8| 2.x and 3.x
-
-.. toctree::
-    :caption: Plugin Developer Documentation
-    :maxdepth: 2
-
-    registering_plugins
-    plugin_parameters
-    formatters
-    cross_compatibility
-
-
-.. _entry points:
-    https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points