Add file to the reno documentation build to show release notes for
stable/train.
Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/train.
Note that release note for train doesn't seem to be ready.
(no 4_0_0_0_hash file exists)
Change-Id: Ia1f9e21af0eab90e1685e9e482fd8affcc415989
Sem-Ver: feature
OpenStack Release Bot
04ed79a7e3