Add GateKeeper instructions for macOS to README

[LittleCube-hax]

Here you go. Let me know if you have any questions or editions you want me to make.

[mondul]

Hi, please note that this only happens in Catalina. Also there is no need to disable gatekeeper, just need to control-click (or tap with two fingers or right-click if you have that configured) each binary, choose open and then choose open again. After that they will be whitelisted and can be used with the helper.

[LittleCube-hax]

That's what I was going to tell them to do originally, but as I was testing again to make sure, it wouldn't work that way. It would only let me do the right-click trick if GateKeeper was already disabled. Note that I used Archive Utility to unzip. The only message that would pop up said "7za is damaged and can't be opened. You should move it to the trash." Because of this, it was my understanding that they had changed the whitelist system.

Unzipping using a third-party application like Keka allowed me to use the right-click trick, and the created binary had the same MD5 as the one Archive Utility created, so the binary itself clearly wasn't broken or even different.

Let me know if you still want me to change it, and I will.