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>

docs/source/index.rst

@@ -83,6 +83,8 @@ The ``master`` branch should be considered experimental and used with caution.
 
 - ``master`` Supports the master branch of Ceph. This branch requires Ansible version ``2.9``.
 
+.. NOTE:: ``stable-3.0`` and ``stable-3.1`` branches of ceph-ansible are deprecated and no longer maintained.
+
 Configuration and Usage
 =======================