Add a test documentation section to the docs

This commit adds a new section the HACKING and REVIEWING docs. It
explains the guidelines around inline documentation when
contributing new tests.

Change-Id: Ib56723d0c748baf41439975102d1363f81085f50
diff --git a/REVIEWING.rst b/REVIEWING.rst
index d6dc83e..74bd2ad 100644
--- a/REVIEWING.rst
+++ b/REVIEWING.rst
@@ -51,6 +51,15 @@
 whether to skip or not.
 
 
+Test Documentation
+------------------
+When a new test is being added refer to the :ref:`TestDocumentation` section in
+hacking to see if the requirements are being met. With the exception of a class
+level docstring linking to the API ref doc in the API tests and a docstring for
+scenario tests this is up to the reviewers discretion whether a docstring is
+required or not.
+
+
 When to approve
 ---------------
  * Every patch needs two +2s before being approved.