Added wkhtmltopdf installation command for macOS

Author: snwfdhmp

README.rst

@@ -28,6 +28,12 @@ Installation
 .. code-block:: bash
 
 	$ sudo apt-get install wkhtmltopdf
+	
+* macOS:
+
+.. code-block:: bash
+
+	$ brew install caskroom/cask/wkhtmltopdf
 
 **Warning!** Version in debian/ubuntu repos have reduced functionality (because it compiled without the wkhtmltopdf QT patches), such as adding outlines, headers, footers, TOC etc. To use this options you should install static binary from `wkhtmltopdf <http://wkhtmltopdf.org/>`_ site or you can use `this script <https://github.com/JazzCore/python-pdfkit/blob/master/travis/before-script.sh>`_.