Add a note to the README about the dangers of cloning to ~/Documents

[cooljeanius]

By default, the GitHub Desktop app clones repositories to ~/Documents/GitHub. This means that if people click the "Open with GitHub Desktop" button, that's where this repository will get put (at least unless the user intervenes). This would normally be fine, however, in recent versions of macOS, the OS has started protecting the ~/Documents folder so that the usual chown/chmod methods of granting permissions to access it are no longer enough. Instead, if the user clones the repository to ~/Documents, they then have to go into System Preferences, click on the "Security & Privacy" prefpane, and then grant Full Disk Access to all the relevant MacPorts binaries (which can be somewhat difficult since the MacPorts prefix is hidden from this prefpane by default; I had to symlink it somewhere where it could find it). There should be some sort of note in the README warning of this potential issue that users could experience.

[RJVB]

WTH... I'm still waiting for the news that you have to start paying Apple a subscription in order to be able to save anything on your computer...

[RJVB]

Fixed with #10a4f5bd12652c82715520ef06eb859955ee45ce