Add 'destroy' method to documentation

[matlembo]

Fantastic plugin.

Just thought you might want to add the 'destroy' option to the documentation as it's really handy for creating on hover parallax effects, for instance, but I had to look through the plugin code to find it!

$('#my-element').hover(
    function(){
        $(this).parallaxify(args);
    },
    function(){
        $(this).parallaxify('destroy');
    }
);

[hwthorn]

Thank you for pointing it out. Just added it to the documentation.