Add REVIEWING.rst

This commit adds a file to outline what to look for as a tempest code
reviewer.

Change-Id: I945469842183d0e5022cd41f823d4892adff333f
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 25bc900..d3118ac 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -9,6 +9,7 @@
 
    overview
    HACKING
+   REVIEWING
 
 ------------
 Field Guides