Add SMC decode README and OSX details

[dan-v]

I bumped into some minor issues while attempting to get the SMC decode script to work, so I wanted to contribute some additional details to help others in the form of a README:

• The existing instructions were at the top of SMC decode script, so to make it easier to find this was moved into the README.
• The existing instructions were Windows specific, but this script works fine on other systems. I tested it on OSX and added a separate section to the README. Also renamed the script from smc_decode_win.py to smc_decode.py
• Added a section to README that shows how to connect the Mini BLE card to the card reader with a photo. My method was a bit destructive, but at least works.

[limpkin]

thanks a lot!