Add document for backchannel UART on Mac

[mike-rogers]

Getting the backchannel UART working on a Mac was somewhat roundabout. A document outlining the process could be very helpful.

Personal notes:

• The drivers for the TI USB Virtual Com Port can be found here.
• Since stty is weird, use screen to connect. Syntax is screen -U /dev/cu.uart<tab> 2400.