Add Arduino AVR Boards 1.6.12 incompatibility warning

Arduino AVR Boards 1.6.12 upgraded avrdude to avrdude 6.3.0-arduino2.
avrdude's handling of unused fuse bits has changed since the previous
version Arduino used(6.0.1-arduino5) which causes the ATmega328P
extended_fuses value to fail verification(see
arduino/Arduino#5175) and the unlock_bits and
lock_bits values to fail verification when used with Atmel AVRISP mkII
and likely any other stk500v2 based programmer(see
arduino/avrdude-build-script#2). Fixing these
issues while maintaining backwards compatibility with previous Arduino
AVR Boards versions would be possible by adding an avrdude tool to
avr_boot but the Arduino developers have decided to revert to the
previous avrdude version on the soon to be released Arduino AVR Boards
1.6.13 and only upgrade to a new avrdude version once it has been
modified to work with the traditional Arduino fuse values(see
arduino/Arduino#5161 and
arduino/Arduino#5202). This makes me think that
the extra complexity of adding an avrdude tool is not worth the benefit
of providing compatibility with a single buggy Arduino AVR Boards
version.

Author: per1234

README.md

@@ -5,6 +5,8 @@ This bootloader loads a file from the SD card on startup. This documentation is
 
 
 ## Installation
+**WARNING: avr_boot is not compatible with Arduino AVR Boards 1.6.12(included with Arduino IDE 1.6.10).**
+
 There are two options for installing **avr_boot** boards in the Arduino IDE:
 #### Boards Manager Installation(requires Arduino IDE version 1.6.4 or greater)
 - Open the Arduino IDE.