Update master for stable/train

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

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

Change-Id: I886cc59de42a5be385ba3a4aebc65c8fbfc021da
Sem-Ver: feature

releasenotes/source/index.rst

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

releasenotes/source/train.rst

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