document known broken "make upload"

The "make upload" is broken. I do not yet know why. --eric
/home/eric/src/Arduino/build/linux/work/hardware/tools/bossac -i -d --port=ttyACM0 -U false -e -w -v -b example_firmware.bin -R
Send auto-baud
Set binary mode
readWord(addr=0)=0x3030306a
readWord(addr=0x400e0740)=0x30303030
chipId=0x30303030
Unsupported processor
No device found on ttyACM0
make: *** [upload] Error 1