Add comments about disabled pep8 rules

setup.cfg

@@ -16,5 +16,17 @@ no-path-adjustment=1
 #pdb=1
 
 [pep8]
+# error codes: http://pep8.readthedocs.org/en/latest/intro.html#error-codes
+# E501: line too long
+# E265: block comment should start with ‘# ‘
+#   We have lots of comments that look like "##### HEADING #####" which violate
+#   this rule, because they don't have a space after the first #. However,
+#   they're still perfectly reasonable comments, so we disable this rule.
+# W602: deprecated form of raising exception
+#   We do this in a few places to modify the exception message while preserving
+#   the traceback. See this blog post for more info:
+#   http://nedbatchelder.com/blog/200711/rethrowing_exceptions_in_python.html
+#   It's a little unusual, but we have good reasons for doing so, so we disable
+#   this rule.
 ignore=E501,E265,W602
 exclude=migrations