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.
Change-Id: I2c780ee3f133f0e37d2a8fdb8a87f17b53992bd1
Sem-Ver: feature
OpenStack Release Bot
fdc5ae3f86