Add Homebrew installation instructions

[ping-localhost]

Since Homebrew is supported by Pie (or Pie at least available on Homebrew), it would be nice if something about it were to be added to the README.md. Not sure if this is wanted, so I thought I would ask before making a PR.

[asgrim]

Brew is not really a supported installation mechanism; and it actually broke things in the pie self-update (see bug #579 - since fixed), so I'm not personally a big fan of this, as it makes extra work :/

[ping-localhost]

Understandable. You're already doing extra work to ensure no weird behaviour happens and by inviting people to use Homebrew, you probably end up having to support more edge cases (e.g. Homebrew can be installed on non-OSX machines).

Kind of figured there was a reason to not include it (otherwise it probably would've already been done), so this solves my question.