Several one-off contributors have submitted patches to Waffle. And they
typically email me directly and send the patches as attachments. Maybe
they didn't bother to read submitting-patches.txt, so let's move it in
a more prominent location.

Rename "/doc/submitting-patches.txt" to "/HACKING.txt". No changes to
file content.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>