Add dhcp_domain option in readme

(was already included in metadata/control/cluster.yml)
diff --git a/README.rst b/README.rst
index b93b061..2424232 100644
--- a/README.rst
+++ b/README.rst
@@ -29,6 +29,7 @@
         cross_az_attach: false
         workers: 8
         report_interval: 60
+        dhcp_domain: novalocal
         bind:
           public_address: 10.0.0.122
           public_name: openstack.domain.com