Update README.md

Add new required role `clusterMonitor` for mongo in `init-mongo.sh`.

See also 
https://community.ui.com/questions/External-Mongodb-question/2ae888c9-978c-4fd6-9032-a80a8b9f96d8

Author: rdehuyss

README.md

@@ -89,7 +89,8 @@ db.createUser({
   roles: [
     { db: "${MONGO_DBNAME}", role: "dbOwner" },
     { db: "${MONGO_DBNAME}_stat", role: "dbOwner" },
-    { db: "${MONGO_DBNAME}_audit", role: "dbOwner" }
+    { db: "${MONGO_DBNAME}_audit", role: "dbOwner" },
+    { db: "admin", role: "clusterMonitor" }
   ]
 })
 EOF