Revise readme

Author: pablosun

mtk/avr/bootloaders/caterina/Readme.txt

@@ -2,7 +2,9 @@ Notes on building LinkIt Smart 7688 Duo ATMega32u4 Caterina bootloader:
 
 1. Must download LUFA-111009(http://www.github.com/abcminiuser/lufa/archive/LUFA-111009.zip) and extract to the same-named directory. Alternatively, if you pulled this repository with git, do a `git submodule update --init` to pull from its repository.
 
-2. The "build.txt" file and "program.txt" files can be renamed
+2. You need avr toolchain (Win-AVR / avr-gcc) to build this bootloader.
+
+3. The "build.txt" file and "program.txt" files can be renamed
     to .bat files and run to build and program the boards listed
     herein (Windows only, unfortunately).