Added note on using older brokers and how to fix the problem.
Added note on using older brokers and how to fix the problem.

Closes #257.

mcanthony/MQTT.js,AdySan/MQTT.js,RangerMauve/MQTT.js,wesley1001/MQTT.js,jdiamond/MQTT.js,jaambee/MQTT.js,bqevin/MQTT.js,itavy/MQTT.js