Document how to set up sleep mode for the SAMD BSP for Adafruit

[terrillmoore]

It's a bit tricky to update the Adafruit BSP with the required sleep-mode changes. Please document.

1. Need the locations on Windows, Mac, Linux.
2. Need the instructions:

   % cd {somewhere}/arduino15/packages/adafruit/hardware/samd/1.0.13
   % git remote add mcci https://github.com/mcci-catena/ArduinoCore-samd.git
   % git pull mcci

[forrestfiller]

@terrillmoore your README.md does a nice job of walking people through this process, you may be able to close this issue at this point: SAMD BSP instructions

[terrillmoore]

Thanks @forrestfiller -- fixed by 7702e64