doc: add a note about deprecated branches

This commit adds a note about `stable-3.0` `stable-3.1` branches which
are deprecated and not maintained anymore.
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit bbe30bcc)

docs/source/index.rst

@@ -77,6 +77,8 @@ The ``master`` branch should be considered experimental and used with caution.
 
 - ``master`` Support for Ceph versions ``luminous``, and ``mimic``. This branch supports Ansible version ``2.6``.
 
+.. NOTE:: ``stable-3.0`` and ``stable-3.1`` branches of ceph-ansible are deprecated and no longer maintained.
+
 Configuration and Usage
 =======================