Set up 3.x release notes

Give the 2.x series its own release notes page, put the 3.x notes into
the 'current' series page.  This removes the OpenStack development cycle
name from the release notes.

We follow the intermediary release model, however the association with
the common cycle release is something that we do not need to reflect in
the doc structure.  We want users to use the current version even with
older clouds, not to think they need to use a 'Mitaka' OSC with a Mitaka
cloud.

Change-Id: Idbb6844824d50e5cd0fa64871b5aa625d4d237af

releasenotes/source/20_releases.rst

@@ -0,0 +1,6 @@
+========================
+2.0 Series Release Notes
+========================
+
+.. release-notes::
+   :version: 2.6.0,2.5.0,2.4.0,2.2.0,2.1.0,2.0.0

releasenotes/source/index.rst

@@ -6,5 +6,28 @@ OpenStackClient Release Notes
    :maxdepth: 1
 
    unreleased
-   mitaka
+   20_releases
    pre_20_releases
+
+OpenStack Releases
+------------------
+
+OpenStackClient is compatible with all currently supported OpenStack releases,
+it does not require maintaining a 'Mitaka' version to match to a Mitala-release
+cloud.  The OpenStackClient release that was current when the corresponding
+OpenStack release was made is shown below:
+
+=================  =======================
+OpenStack Release  OpenStackClient Release
+=================  =======================
+Mitaka             2.3.0
+Liberty            1.7.3
+Kilo               1.0.6
+Juno               0.4.1
+Icehouse           0.3.1
+=================  =======================
+
+Further details for historical OpenStack releases are found at the
+`OpenStack Releases`_ page.
+
+.. _`OpenStack Releases`: http://releases.openstack.org/

releasenotes/source/mitaka.rst

@@ -1,6 +0,0 @@
-===========================
-Mitaka Series Release Notes
-===========================
-
-.. release-notes::
-   :branch: origin/stable/mitaka

releasenotes/source/unreleased.rst

@@ -3,3 +3,4 @@ Current Release Notes
 =====================
 
 .. release-notes::
+   :earliest-version: 3.0