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: I167c9ab4adc683e8d6dbabf68181129c4b865954

releasenotes/source/index.rst

@@ -19,6 +19,7 @@
    :maxdepth: 2
 
    unreleased
+   wallaby
    victoria
    ussuri
    train

releasenotes/source/wallaby.rst

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