add hacking preamble

The current hacking rules read too much like the 10 commandments,
with no context. Context is important.

Change-Id: Id10897eebe3cdb8d00a12b453d1133d2f8768d85

HACKING.rst

@@ -1,10 +1,22 @@
 OpenStack Style Guidelines
 ==========================
 
+OpenStack has a set of style guidelines for clarity. OpenStack is a
+very large code base (over 1 Million lines of python), spanning dozens
+of git trees, with over a thousand developers contributing every 12
+months. As such common style helps developers understand code in
+reviews, move between projects smoothly, and overall make the code
+more maintainable.
+
+
+Step 0
+------
+
 - Step 1: Read http://www.python.org/dev/peps/pep-0008/
 - Step 2: Read http://www.python.org/dev/peps/pep-0008/ again
 - Step 3: Read on
 
+
 General
 -------
 - [H903] Use only UNIX style newlines (``\n``), not Windows style (``\r\n``)

README.rst

@@ -41,6 +41,19 @@ Each check is a pep8 plugin so read
 
 - https://github.com/jcrocholl/pep8/blob/master/docs/developer.rst#contribute
 
+The focus of new or changed rules should be to do one of the following
+
+- Substantially increase the reviewability of the code (eg: H301,2,3
+  as they make it easy to understand where symbols come from)
+- Catch a common programming error that may arrise in the future (H201)
+- Prevent a situation that would 100% of the time be -1ed by
+  developers (H903)
+
+But, as always, remember that these are Guidelines. Treat them as
+such. There are always times for exceptions. All new rules should
+support noqa.
+
+
 Requirements
 ------------
 - The check must already have community support. We do not want to dictate style, only enforce it.