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.
Actually, I should fix the metadata, so I'll do this at the same time.