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: I376bba8692bd6befa53057ab2110dd74458092b2

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    yoga
     xena
     wallaby
     victoria

releasenotes/source/yoga.rst

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