Update master for stable/xena

Add file to the reno documentation build to show release notes for
stable/xena.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/xena.

Sem-Ver: feature
Change-Id: Ie1e7be2d3c88b4ba376d4d6a2f4c8ee01a752dcd