Add HACKING.rst

HACKING.rst is a convention used by OpenStack python packages,
let's use it here as well.

Change-Id: I6df085d206efbb43b61b8ee0cfc187462c8e9d26

HACKING.rst

@@ -0,0 +1,4 @@
+openstack-doc-tools Style Commandments
+======================================
+
+Read the OpenStack Style Commandments http://docs.openstack.org/developer/hacking/