The CONTRIBUTING.rst is now included among the files
triggering execution of the tox-docs job.
This patches a dimunitive but nonetheless extant hole
in our doc CI coverage.
Signed-off-by: Jiri Podivin <jpodivin@redhat.com>
Change-Id: I96fa87d83bfa9a5f48812c0d15504e80d4548d53