Adding explicit UNLICENSE file

Since the code reads *"Released into the public domain."* (ever since the first commit, see https://github.com/miguelbalboa/rfid/commit/54e32172614cbc77db0c3d74dade5b16f0d512f5) it seems that the guidance at http://unlicense.org/ could be applied to make it more explicit. That is what this PR implements.