Add flake8.api and docs for it

Stub out the primary method people seem to use.

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
+    python-api
 
 .. config files
 .. command-line tutorial

docs/source/user/python-api.rst

@@ -0,0 +1,11 @@
+===================
+ Public Python API
+===================
+
+Flake8 3.0.0 presently does not have a public, stable Python API.
+
+When it does it will be located in :mod:`flake8.api` and that will
+be documented here.
+
+.. automodule:: flake8.api
+    :members:

flake8/api.py

@@ -0,0 +1,10 @@
+"""Module containing all public entry-points for Flake8.
+
+This is the only submodule in Flake8 with a guaranteed stable API. All other
+submodules are considered internal only and are subject to change.
+"""
+
+
+def get_style_guide(**kwargs):
+    """Stub out the only function I'm aware of people using."""
+    pass