Add installation instructions using Homebrew

I was not aware there was a [Homebrew cask](https://github.com/Homebrew/homebrew-cask/blob/master/Casks/cool-retro-term.rb) for this
project, which simplifies installation on macOS. This commit adds
documentation for installing the app via Homebrew.

README.md

@@ -202,6 +202,12 @@ cp -r qmltermwidget/QMLTermWidget cool-retro-term.app/Contents/PlugIns
 open cool-retro-term.app
 ```
 
+**Homebrew**
+
+```sh
+brew cask install cool-retro-term
+```
+
 ## Donations
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