Clarify that subscribe() must be called before connect()
Clarify that subscribe() must be called before connect()

georgeartem/Arduino_MQTT,AdySan/Adafruit_MQTT_Library,Marzogh/Adafruit_MQTT_Library,spark/Adafruit_MQTT_Library,milk-cocoa/Adafruit_MQTT_Library,adafruit/Adafruit_MQTT_Library