Update master for stable/wallaby

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

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

Sem-Ver: feature
Change-Id: Id1452355df143f575d140ff3edab84ab3d919ec2

releasenotes/source/index.rst

@@ -9,6 +9,7 @@ Contents
    :maxdepth: 2
 
    unreleased
+   wallaby
    victoria
    ussuri
    train

releasenotes/source/wallaby.rst

@@ -0,0 +1,6 @@
+============================
+Wallaby Series Release Notes
+============================
+
+.. release-notes::
+   :branch: stable/wallaby