Adding selfHost documentation for mqtt

[egekorkan]

Currently, the MQTT binding allows a convenience function to self host the mqtt broker. This is visible in https://github.com/eclipse/thingweb.node-wot/blob/master/packages/binding-mqtt/src/mqtt-broker-server.ts#L69 however it is not documented for the users to try it out.

Also it seems that uri is mandatory now but the example in the readme is not up to date

Something like this should be put in the config of the Readme example:

servient.addServer(new MqttBrokerServer({
    uri:"localhost:1883",
    selfHost: true
}));