Add directory for Queens spec

We reached Cinder Spec Freeze [1] for Pike, so all specs should be
proposed to the next release.

[1]
https://releases.openstack.org/pike/schedule.html#p-cinder-spec-freeze

Change-Id: I308479d5d34edc01d2c6571883a5cae5f10b5c30

doc/source/index.rst

@@ -4,6 +4,14 @@
 Volume Service Specifications (cinder)
 ======================================
 
+Queens approved specs:
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+    specs/queens/*
+
 Pike approved specs:
 
 .. toctree::