Update master for stable/stein

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

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

Change-Id: I15dc275f59b165f20a8eafb40c1b0940cd322168
Sem-Ver: feature

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    stein
     rocky
     queens
     pike

releasenotes/source/stein.rst

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