Documentation: Add class 'gridster-no-drag' on element to disable draggable.

[natee]

angular-gridster allows us to disable draggable when click some elements, but there were no documents about how to use it.

1. Add a class 'gridster-no-drag'

    // only works if you have jQuery
    if ($target.closest && $target.closest('.gridster-no-drag').length) {
        return false;
    }

2. Add a directive

   .directive('gridsterNoDrag', function() {
   return {
       restrict: 'A',
       link: function(scope, $element) {
           $element.addClass('gridster-no-drag');
       }
   };
   })