Add documentation on command deprecation process

Since we haven't removed deprecated commands before, add some
documentation to the devref on the process for this type of change.

Change-Id: Iff6f5df372bec524757b84747c7a95bbf6412c35

doc/source/index.rst

@@ -48,6 +48,37 @@ See `Consistent Testing Interface`_ for more details.
 
 .. _Consistent Testing Interface: http://git.openstack.org/cgit/openstack/governance/tree/reference/project-testing-interface.rst
 
+Deprecating commands
+====================
+
+There are times when commands need to be deprecated due to rename or removal.
+The process for command deprecation is:
+
+  1. Push up a change for review which deprecates the command(s).
+
+   - The change should print a deprecation warning to stderr each time a
+     deprecated command is used.
+   - That warning message should include a rough timeline for when the command
+     will be removed and what should be used instead, if anything.
+   - The commit message on the change should include a DocImpact tag so it
+     gets in the release notes.
+   - The deprecation cycle is typically the first client release *after* the
+     next *full* Nova server release so that there is at least six months of
+     deprecation.
+
+  2. Once the change is approved, have a member of the `nova-release`_ team
+     release a new version of python-novaclient.
+
+     .. _nova-release: https://review.openstack.org/#/admin/groups/147,members
+
+  3. Example: `<https://review.openstack.org/#/c/185141/>`_
+
+     This change was made while the Nova 2015.2 Liberty release was in
+     development. The current version of python-novaclient at the time was
+     2.25.0. Once the change was merged, python-novaclient 2.26.0 was released.
+     Since there was less than six months before 2015.2 would be released, the
+     deprecation cycle ran through the 2016.1 Nova server release.
+
 Indices and tables
 ==================