Document standalone + py26 aims

"This library aims to support Python 2.6 and work as a standalone script
with no dependencies."

#455 (comment)

Author: doismellburning

docs/developer.rst

@@ -31,6 +31,8 @@ Some high-level aims and directions to bear in mind for contributions:
 * If you want to provide extensibility / plugins,
   please see `flake8 <https://gitlab.com/pycqa/flake8>`_ -
   ``pep8`` doesn't want or need a plugin architecture.
+* Python 2.6 support is still deemed important.
+* ``pep8`` aims to have no external dependencies.
 
 
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