Add helper note on MacOS quarantine for "unknown" executables

[akabiru]

https://support.apple.com/en-is/102445

MacOS will place the executable in quarantine by default. If your Mac displays an alert when you run the executable, you can dequarantine the op executable via the following command:

xattr -d com.apple.quarantine /usr/local/bin/op

[Kharonus]

Thanks a lot kabiru for the contribution. :D Mac was not yet much of a testing ground.