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