Merge pull request #1767 from demoneaux/readme

Add note about head.js to readme.

README.md

@@ -363,6 +363,7 @@ You can add your own extensions using the same syntax. The following properties
 - **callback**: [optional] Function to execute when the script has loaded
 - **condition**: [optional] Function which must return true for the script to be loaded
 
+To load these dependencies, Reveal.js requires [head.js](http://headjs.com/) *(a script loading library)* to be loaded before Reveal.js.
 
 ### Ready Event