Fix doc: Issue with OSX Catalina

[AarneHuttunen]

When installing on OSX Catalina, the system prevents installation by double clicking and the basic right click open doesn't work either.

The correct way to install is to open system preferences > Security & Privacy and click the allow button on the screen.

[TheRealWaldo]

Installation worked for me without having to allow; however, it shows 'You cannot use this Matrix Screen Saver with this version of MacOS' for the screen saver itself!