Added recommendation against ast usage in pep8

Based on PyCQA#431 (comment)

Author: doismellburning

docs/developer.rst

@@ -25,7 +25,9 @@ Direction
 
 Some high-level aims and directions to bear in mind for contributions:
 
-
+* ``pep8`` is intended to be as fast as possible.
+  Using the ``ast`` module defeats that purpose.
+  The `pep8-naming <https://github.com/flintwork/pep8-naming>`_ plugin exists for this sort of functionality.
 
 
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