Update master for stable/yoga

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

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

Sem-Ver: feature
Change-Id: I94d459d295715fac20b7d81380a3f2c77966f0cd

releasenotes/source/index.rst

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

releasenotes/source/yoga.rst

+=========================
+Yoga Series Release Notes
+=========================
+
+.. release-notes::
+   :branch: stable/yoga