edison.md: Add docs for MFD2

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docs/edison.md

@@ -37,6 +37,10 @@ in libmraa:
   of the Arduino breakout board, this is not supported officially so asking for
   mraa_i2c_init(1) will result in getting i2c bus 6 (the default one). However
   using raw mode (mraa_i2c_init_raw(1)) this bus is fully usable
+- If you want to use /dev/ttyMFD2 you have to use the raw uart mode passing a
+  std::string or char* argument of "/dev/ttyMFD2" to mraa:Uart() or
+  mraa_uart_init_raw. By default there is a getty running on that interface so
+  you will need to disable that first
 
 Because of the way IO is setup with the tristate on the Arduino breakout board
 IO will be flipped as it is setup. It's recommended to setup IO pins &