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: I721bcefb66864d2c281f0e936b13531403aa0959

releasenotes/source/index.rst

@@ -6,3 +6,4 @@ oslo.metrics Release Notes
    :maxdepth: 1
 
    unreleased
+   wallaby

releasenotes/source/wallaby.rst

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