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: I4537cf2c4fb53095d8592bdea0f431e2a5393c4a

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    wallaby
     victoria
     ussuri
     train

releasenotes/source/wallaby.rst

+============================
+Wallaby Series Release Notes
+============================
+
+.. release-notes::
+   :branch: stable/wallaby