Add a note about the cluster node roles

Change-Id: I928629ef1de0d082280fe1b8dff90c20e303bd18
diff --git a/edge_deployment/runtime_deployment.rst b/edge_deployment/runtime_deployment.rst
index 8fb68f9..231b0a7 100644
--- a/edge_deployment/runtime_deployment.rst
+++ b/edge_deployment/runtime_deployment.rst
@@ -66,6 +66,12 @@
    Created ../production/ace-test/backend.tf
    Created ../production/ace-test/cluster_val.tfvars
 
+.. attention::
+
+  If the cluster has an even number of compute nodes, edit **cluster_val.tfvars**
+  file so that only the odd number of nodes have **etcd** and **controlplane**
+  roles.
+
 Create a review request.
 
 .. code-block:: shell