docs: Add Mac OS installation instructions

See https://github.com/adrienverge/yamllint/issues/91 and
https://github.com/Homebrew/homebrew-core/blob/af2bbe9/Formula/yamllint.rb

README.rst

@@ -50,6 +50,12 @@ On Debian 8+ / Ubuntu 16.04+:
 
  sudo apt-get install yamllint
 
+On Mac OS 10.11+:
+
+.. code:: bash
+
+ brew install yamllint
+
 Alternatively using pip, the Python package manager:
 
 .. code:: bash

docs/quickstart.rst

@@ -16,6 +16,12 @@ On Debian 8+ / Ubuntu 16.04+:
 
  sudo apt-get install yamllint
 
+On Mac OS 10.11+:
+
+.. code:: bash
+
+ brew install yamllint
+
 Alternatively using pip, the Python package manager:
 
 .. code:: bash