Add notice about MQTT 3.1.1 broker
Add notice about MQTT 3.1.1 broker

A list of supported brokers would be a nice addition.
doebi/pubsubclient,Imroy/pubsubclient,liquiddandruff/pubsubclient,liquiddandruff/pubsubclient,liquiddandruff/pubsubclient,vshymanskyy/pubsubclient,doebi/pubsubclient,Imroy/pubsubclient,Imroy/pubsubclient,doebi/pubsubclient,hemantsangwan/Arduino-PubSubClient,koltegirish/pubsubclient,Protoneer/pubsubclient,hemantsangwan/Arduino-PubSubClient,vshymanskyy/pubsubclient,Protoneer/pubsubclient,hemantsangwan/Arduino-PubSubClient,vshymanskyy/pubsubclient,Protoneer/pubsubclient,koltegirish/pubsubclient,koltegirish/pubsubclient