Update master for stable/ussuri

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

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

Change-Id: Ic22d1f86c4527bb7153b18bf395d82a36f149d68
Sem-Ver: feature

releasenotes/source/index.rst

@@ -8,6 +8,7 @@ Contents
    :maxdepth: 2
 
    unreleased
+   ussuri
    train
    stein
    rocky

releasenotes/source/ussuri.rst

+===========================
+Ussuri Series Release Notes
+===========================
+
+.. release-notes::
+   :branch: stable/ussuri