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: I2140ba315f0cd83f89fe2b3d7529a5b016b3ce03

releasenotes/source/index.rst

@@ -6,4 +6,5 @@ oslo.metrics Release Notes
    :maxdepth: 1
 
    unreleased
+   xena
    wallaby

releasenotes/source/xena.rst

+=========================
+Xena Series Release Notes
+=========================
+
+.. release-notes::
+   :branch: stable/xena