Adjust standalone example so it works

As per #47 `memory` is missing from the standalone example. I hit this issue and it stopped me for a while before I figured it out.

This is a PR to make the adjustment, and is what I did to get it working.

Change-Id: Icf98f4096a2305b65b1339330db379a48ac93dd6
diff --git a/README.rst b/README.rst
index bab7652..e995784 100644
--- a/README.rst
+++ b/README.rst
@@ -17,6 +17,8 @@
     rabbitmq:
       server:
         enabled: true
+        memory:
+          vm_high_watermark: 0.4
         bind:
           address: 0.0.0.0
           port: 5672