This change adds a description how to use reno within a travis CI
setup. It's neither a bug in Travis nor in reno. It's just a
different set of expectations of these two. As reno gets used
outside of OpenStack too, it makes sense to clarify the usage.
Change-Id: I675402402d5eb3d9afd374c149b21c75977946c7
Related-bug: 1703603
e5b7dae85e