Adding note about Arduino VID and PIDs.

Author: damellis

hardware/arduino/firmwares/README.txt

@@ -11,4 +11,7 @@ To burn (Uno):
 avrdude -p at90usb82 -F -P usb -c avrispmkii -U flash:w:UNO-dfu_and_usbserial_combined.hex -U lfuse:w:0xFF:m -U hfuse:w:0xD9:m -U efuse:w:0xF4:m -U lock:w:0x0F:m
 
 To burn (Mega 2560):
-avrdude -p at90usb82 -F -P usb -c avrispmkii -U flash:w:MEGA-dfu_and_usbserial_combined.hex -U lfuse:w:0xFF:m -U hfuse:w:0xD9:m -U efuse:w:0xF4:m -U lock:w:0x0F:m
+avrdude -p at90usb82 -F -P usb -c avrispmkii -U flash:w:MEGA-dfu_and_usbserial_combined.hex -U lfuse:w:0xFF:m -U hfuse:w:0xD9:m -U efuse:w:0xF4:m -U lock:w:0x0F:m
+
+Please note that the Arduino VID and PIDs are only for use with official
+Arduino hardware and should not be used on other products.