Add `brew` instructions for mac (#2108)
Add `brew` instructions for mac (#2108)

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

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