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: I0f76f6a1b052d8cf7e1a27697cfb9cce2fd4475d

releasenotes/source/index.rst

@@ -5,6 +5,7 @@ oslo.upgradecheck Release Notes
    :maxdepth: 1
 
    unreleased
+   yoga
    xena
    wallaby
    victoria

releasenotes/source/yoga.rst

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