Public review comment received from Nicholas Humfrey (point 1)

1) I noticed that the previous version of the specification is listed
in the references (MQTTV31) but it not actually mentioned in the text
anywhere. My reason for looking it up is that it would be extremely
useful to have a list of differences between versions 3.1.1 and 3.1.0 of
the specification. I started compiling my own list on the mqtt.org wiki:

https://github.com/mqtt/mqtt.github.io/wiki/Differences-between-3.1.0-and-3.1.1