Add CONTRIBUTING.rst intructions

CONTRIBUTING.rst

+Jenkins
+=======
+
+Jenkins build public changeset every day at 4 AM on https://jenkins.intra.logilab.fr/job/docker-cubicweb/
+
+README on docker hub
+====================
+
+To generate the README for docker hub::
+
+   pandoc --from rst --to gfm README.rst