Update cygwin-build.txt

Changed line 61 to "git clone git://github.com/sgminer-dev/sgminer.git" (Updated to the new address "sgminer-dev" instead of "veox", and changed to "git://" instead of "git@" because it is way easier for newbies to download it that way).

Also added a warning at line 75 that users MUST download the ADL_SDK version 6.0 because v7.0 won't work with SGMiner 4.2.2