Add instructions how to get unsigned binaries

[lidel]

#367 introduced macos signing, which appends signature to the end of binary.

Would be useful to expand https://github.com/ipfs/distributions/blob/master/README.md#notes-on-reproducible-builds section with info how to undo signing and get the original binary, so people working on reproducible builds can wire up verification with more ease.