Add BFx2 USB miner instructions to README.ASIC

Using this miner is not obvious since it's necessary to install the WinUSB driver and then pass the -S bfx:all option. I have taken the instructions from the bitcointalk thread (https://bitcointalk.org/index.php?topic=524521.msg6194413) and added them to the README.ASIC file.
Can you take this line out? I think the rest covers it all...