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/hacking.rst b/doc/source/hacking.rst
new file mode 100644
index 0000000..a2bcf4f
--- /dev/null
+++ b/doc/source/hacking.rst
@@ -0,0 +1 @@
+.. include:: ../../HACKING.rst