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: I13cd4f408756daeca29110062d0ffce327c0ae12
Sem-Ver: feature

releasenotes/source/index.rst

@@ -22,6 +22,7 @@ Manila Release Notes
    :maxdepth: 1
 
    unreleased
+   train
    stein
    rocky
    queens

releasenotes/source/train.rst

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