Add installation instructions for Homebrew
Add installation instructions for Homebrew

Includes instructions for a `HEAD` installation, which builds the latest `master`.

SethTisue/scalariform,pinghe/scalariform,dastbe/scalariform,kahosato/scalariform,pinghe/scalariform,triggerNZ/scalariform,pvlugter/scalariform,kahosato/scalariform,daniel-trinh/scalariform,pvlugter/scalariform,dastbe/scalariform,daniel-trinh/scalariform,SethTisue/scalariform,triggerNZ/scalariform