Mention J2 use for serial communication through USB on chipignite Readme

[mbalestrini]

I think it would be useful to mention that J2 on the development board connects Caravel UART to the FTDI chip on: https://github.com/efabless/caravel_board/blob/8b965cd39c64f28c24af80159d6b48bc4cc00404/firmware/chipignite/README.md

Maybe something like this?

Shorting J2 enables the serial communication between Caravel's UART and the on board FTDI USB chip.
A serial terminal program like screen can then be used to exchange information between the computer and Caravel.

[mbalestrini]

Maybe adding that while J2 is shorted you can't flash the firmware? (at least that is what it happened in my tests)