Added explanation on how to set will information.
Added explanation on how to set will information.

Fix #263.

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