Add Homebrew install intructions (#106)

README.md

@@ -32,8 +32,14 @@ For Macs without Secure Enclaves, you can configure a Smart Card (such as a Yubi
 
 ### Installation
 
+#### Direct Download
+
 You can download the latest release over on the [Releases Page](https://github.com/maxgoedjen/secretive/releases)
 
+#### Using Homebrew
+
+    brew cask install secretive
+
 ### FAQ
 
 There's a [FAQ here](FAQ.md).