Add debugging topic

[hamishwillee]

Seed with information here: https://github.com/dronecore/DroneCore/pull/150#issuecomment-343299088

Add info on developer logging - ie both messages you can add to plugins, and the message logging for all MAVLink comms: https://docs.dronecore.io/v/develop/en/contributing/dev_logging.html#message_logging

Perhaps start a FAQ on how to deal with particular types of issues - e.g. "What do to if you get a timeout" - with "ways to check that message is actually sent".

For this, from Julian - to solve this problem (timeout)

• You need to know that a mavlink command gets published as a uORB vehicle_command
• And that a vehicle_command_ack as a mavlink command ack again