Add homebrew cask install method to README

[brandonb927]

I typed out a lengthy description before even attempting it, but you can install this via homebrew cask!

It would be great to have this in the README 😄

[manosim]

Hello @brandonb927 ! Yes it has been a while since it was added to cask (it was like ages ago - I even forgot about it). Will open a PR soon - feel free to open one yourself! 😄