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: I05f4b8da2e27d3b6c35b0f80edc19e6592c67de2

doc/source/index.rst

@@ -7,6 +7,14 @@
  Oslo Design Specifications
 ============================
 
+Train approved specs:
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   specs/train/*
+
 Stein
 =====
 

specs/train/index.rst

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

specs/train/placeholder.rst

@@ -0,0 +1,124 @@
+..
+ 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 ``oslo-specs`` repository.
+
+Problem description
+===================
+
+Sphinx toctree complains if no file exists in a directory
+specified in toctree glob.
+
+Use Cases
+---------
+
+None
+
+Proposed change
+===============
+
+Add this file. Please remove later when the another spec is approved.
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Upgrade impact
+--------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+None
+
+Work Items
+----------
+
+None
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+None
+
+
+Documentation Impact
+====================
+
+None
+
+References
+==========
+
+None
+
+
+History
+=======
+
+None