Include MQTT data in the Input API documentation

[borpin]

The input API documentation only covers the HTTP API and not the MQTT interface.

As this is part of getting data into Emoncms, it seems reasonable to document the input here.

[alexandrecuer]

I dont know if this can be of any help, but I tried some time ago to gather some info on the mqtt and feedwriting services : https://github.com/dromotherm/sandbox/blob/master/bios/servicesEN.md