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

docs/getting-started/index.mdx

@@ -19,7 +19,7 @@ To see app specific data, you need to integrate the Flipper SDK into your app. S
 
 <OssOnly>
 
-The desktop part of Flipper doesn't need any particular setup. Simply download the latest build for [Mac](https://www.facebook.com/fbflipper/public/mac), [Linux](https://www.facebook.com/fbflipper/public/linux) or [Windows](https://www.facebook.com/fbflipper/public/windows) and launch it.
+The desktop part of Flipper doesn't need any particular setup. Simply download the latest build for [Mac](https://www.facebook.com/fbflipper/public/mac), [Linux](https://www.facebook.com/fbflipper/public/linux) or [Windows](https://www.facebook.com/fbflipper/public/windows) and launch it. If you're on macOS, you can run `brew install --cask flipper` to let `homebrew` manage installation and upgrades (simply run `brew upgrade` to upgrade when a new version is released, although it might take a few hours up to a day for the package to be upgraded on `homebrew`). 
 
 In order to work properly, Flipper requires a working installation of the Android and (if where applicable) iOS development tools on your system, as well as the [OpenSSL](https://www.openssl.org) binary on your `$PATH`.