Add placeholder for Train specs

The "T" release of OpenStack is officially "Train".

http://lists.openstack.org/pipermail/openstack-dev/2018-November/136464.html

Change-Id: I8173bd1886df4d306c10d434e2ecdec095070dbd

doc/source/index.rst

@@ -10,6 +10,7 @@ Specifications and RFEs
 .. toctree::
    :maxdepth: 2
 
+   specs/train/index
    specs/stein/index
    specs/rocky/index
    specs/queens/index

specs/train/index.rst

@@ -0,0 +1,9 @@
+=====
+Train
+=====
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   *

specs/train/placeholder.rst

@@ -0,0 +1,31 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==================
+(Placeholder Spec)
+==================
+
+This file is just a placeholder for the specs directory.
+It will be removed soon after some spec is merged.
+
+The latest spec template is found at ``specs/template.rst``
+in the ``neutron-specs`` repository.
+
+Problem Description
+===================
+
+Sphinx toctree complains if no file exists in a directory
+specified in toctree glob.
+
+Proposed Change
+===============
+
+Add this file.
+
+References
+==========
+
+None.