Update README example code.

Author: jburhenn

README.rst

@@ -52,6 +52,8 @@ Below is an example of a simple client that publishes some sample data and recei
   client = cayenne.client.CayenneMQTTClient()
   client.on_message = on_message
   client.begin(MQTT_USERNAME, MQTT_PASSWORD, MQTT_CLIENT_ID)
+  # For a secure connection use port 8883 when calling client.begin:
+  # client.begin(MQTT_USERNAME, MQTT_PASSWORD, MQTT_CLIENT_ID, port=8883)
 
   i=0
   timestamp = 0