[Ovmsdev] Addition to OVMS Developer Guide

[Stephen Casner]

At Mark's request I have added a description of how to add commands to
the console command interpreter including the usage string syntax.
This should be considered a draft at this point; questions and
comments are welcome.

I added this under Vehicle Firmware Overview, but that might not be
the right place.  There is no general description of the core
firmware.

                                                        -- Steve