We need a release notes page to show notes for unreleased versions by
scanning the "current" branch. This ensures that notes files cannot be
merged if they break the releasenotes job.
See also http://lists.openstack.org/pipermail/openstack-dev/2015-November/078940.html
Change-Id: Iaef8b78802c04ec8306dd6f1da02d6c6a50697d8
Mostly cargo-culted from I9549e6b491b26232f4dac900b0bf05f10eb0e30a.
Note that reno automatically inserts version numbers; liberty.rst is
sufficient for intermediate release models.
Change-Id: Ic700d719242b17532b0b43a68b7eff507cb3a43c