Added explanation to config/postal.example.yml about #725 (#920)

config/postal.example.yml

@@ -31,6 +31,11 @@ message_db:
 
 rabbitmq:
   # Specify the connection details for your RabbitMQ server.
+  # if you have more than one RabbitMQ server (cluster), you can use array like this:
+  # host:
+  #   - 10.1.1.1
+  #   - 10.1.1.2
+  #   - 10.1.1.3
   host: 127.0.0.1
   username: postal
   password: p0stalpassw0rd