feat: add homebrew installation note (#468)

Summary:
closes #312

relates to Homebrew/homebrew-core#162375

Pull Request resolved: #468

Differential Revision: D57621792

Pulled By: hick209

fbshipit-source-id: a62db1c685fc318e14708d86b21c0b667033337a

README.md

@@ -54,6 +54,16 @@ Not all of ktfmt's rules can be represented as IntelliJ editor settings, so you
 need to run ktfmt. Alternately, that file can be used as a reference to manually change
 the project's code style settings.
 
+### Installation
+
+#### Homebrew
+
+If you're a [Homebrew](https://brew.sh) user, you can install [ktfmt](https://formulae.brew.sh/formula/ktfmt) via:
+
+```
+$ brew install ktfmt
+```
+
 ### from the command-line
 
 [Download the formatter](https://github.com/facebook/ktfmt/releases)