Update README for macOS installation

[plroebuck]

Never could get growlnotify to work on macOS High Sierra.

Swap order in README to make growlnotify the secondary option to enable package support; at least terminal-notifier is actively maintained...

[plroebuck]

ping @deiga

[deiga]

@plroebuck please make a PR

[plroebuck]

Six months to get a reply to do it myself... which I already had.

[deiga]

@plroebuck Sure, you made it yourself, but I'm asking if you could make a PR about it.

And I'm sorry, my time is limited and I chose not to prioritise this over something else, so it took a while longer to react.