Move contributing into HACKING

contributing.rst doesn't add very much over the extant HACKING.rst, so
move some of the unique bits into HACKING.rst and then link that into
the documentation.

Change-Id: I0530f38eda92f8dd374c0ec224556ace6e679f54
diff --git a/doc/source/index.rst b/doc/source/index.rst
index bac593d..0ffb15c 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -12,7 +12,7 @@
    plugins
    faq
    changes
-   contributing
+   hacking
 
 Quick Start
 -----------
@@ -139,7 +139,7 @@
 Contributing
 ------------
 
-:doc:`Pitching in to make DevStack a better place <contributing>`
+:doc:`Pitching in to make DevStack a better place <hacking>`
 
 Code
 ====