Mac OS install: add Homebrew Cask instructions

INSTALL.asciidoc

@@ -231,14 +231,29 @@ Then <<tox,install qutebrowser via tox>>.
 On OS X
 -------
 
+Prebuilt binary
+~~~~~~~~~~~~~~~
+
 The easiest way to install qutebrowser on OS X is to use the prebuilt `.app`
 files from the
 https://github.com/The-Compiler/qutebrowser/releases[release page].
 
+This binary is also available through the
+https://caskroom.github.io/[Homebrew Cask] package manager:
+
+----
+$ brew cask install qutebrowser
+----
+
+Manual Install
+~~~~~~~~~~~~~~
+
 Alternatively, you can install the dependencies via a package manager (like
 http://brew.sh/[Homebrew] or https://www.macports.org/[MacPorts]) and run
 qutebrowser from source.
 
+==== Homebrew
+
 For Homebrew, a few extra steps are necessary since Homebrew dropped QtWebKit
 from Qt 5.6 - however, some users reported this didn't work for them, so using
 the `.app` is strongly encouraged.
@@ -255,6 +270,8 @@ $ ln -s /usr/local/opt/qt55 /usr/local/opt/qt5
 $ pip3.5 install qutebrowser
 ----
 
+==== MacPorts
+
 For MacPorts, run:
 
 ----