add installation info for later versions of Mac OS

[christiansievers]

Don't want to do the whole shebang with forking and pull request for one paragraph, so please let me propose a simple change to the README here. Please add the following to the instructions for Mac OS

Note on newer Mac OS versions: Increasingly, launching downloaded applications from sources other than the App Store does not work out of the box. If double-clicking the downloaded installer JAR doesn't work, even if you have Java installed and working, follow the instructions for Linux below.

[carlwilson]

Will do, in the next version.