Create HACKING.rst for elements

First item - idempotency of os-refresh-config scripts.

Change-Id: Idcf34021f5ec6fce80dec840d4eb37fecb75194b

HACKING.rst

@@ -0,0 +1,24 @@
+TripleO Style Guidelines
+========================
+
+- Step 1: Read the OpenStack Style Guidelines [1]_.
+- Step 2: Read the tripleo-incubator HACKING.rst [2]_.
+- Step 3: Read on.
+
+Element Specific Guidelines
+---------------------------
+
+- Idempotency. A new version of metadata can be pushed at any time, for example
+  due to a `heat stack-update`. Elements' os-refresh-config scripts must handle
+  this gracefully. If they cannot be fully idempotent, they must fence their
+  once-only sections.
+
+  For example, the keepalived element's configure.d script either reloads or
+  restarts the service based on whether it appears to be already running.
+  Another method of fencing would be to write a marker to ephemeral storage on
+  first execution and skip once-only sections if the marker is present.
+
+References
+----------
+.. [1]  http://docs.openstack.org/developer/hacking/
+.. [2]  http://docs.openstack.org/developer/tripleo-incubator/HACKING.html