Summary:
Thought I'd add this to the getting started, since so many people end up downloading the release directly.
## Changelog
Added `brew` instructions for mac
Pull Request resolved: https://github.com/facebook/flipper/pull/2108
Test Plan: Just docs, so no testing needed
Reviewed By: mweststrate
Differential Revision: D27398664
Pulled By: passy
fbshipit-source-id: be9cc7007e7c5dc717aad60ec0591d4ca0d100cb
26aef92dc5