Allow specifying local engine for client-side storage

If backupninja backs up the database to the local storage
there is no need to use any engine, no engine config file needs
to be created.
Change-Id: Ie1274a63955bcc6f623c46324a3f34f8299a32e8
diff --git a/README.rst b/README.rst
index 8a29afa..cf21103 100644
--- a/README.rst
+++ b/README.rst
@@ -73,6 +73,16 @@
             key: ssh-key
 
 
+Backup client with local storage
+
+.. code-block:: yaml
+
+    backupninja:
+      client:
+        enabled: true
+        target:
+          engine: local
+
 More information
 ================