Prepare Ocata spec directory

Newton feature freeze has happened.
It is time to add a spec directory for Ocata release.

Change-Id: I63faf4f05acb85a86690980d9a33420d484fbf4e

doc/source/index.rst

@@ -4,6 +4,15 @@
 Neutron Project Specifications
 ==============================
 
+Ocata
+=====
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   specs/ocata/*
+
 Newton
 ======
 

specs/ocata/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 Ocata specs directory.
+It will be removed soon after some spec for Ocata 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.