Update README with home-brew cask install instructions

[booc0mtaco]

RE: #7 , it looks like @phinze & co. have updated cask already! How about adding a bit of installation instructions for home-brew? I cloned and updated myself, but could not post the pull request :)

Some verbiage and the command that kicks everything off: brew cask install cloudytabs ?

[josh-]

Excellent idea – I've just done so. Thanks for the suggestion!