docs(API.md): add API.md explaining ui.item.sortable properties

Author: thgreasi

API.md

@@ -0,0 +1,57 @@
+# ui.item.sortable API documentation
+
+## Properties
+
+### dropindex
+Type: [Integer](http://api.jquery.com/Types/#Integer)  
+Holds the index of the drop target that the dragged item was dropped.
+
+
+### droptarget
+Type: [jQuery](http://api.jquery.com/Types/#jQuery)  
+Holds the ui-sortable element that the dragged item was dropped on.
+
+### droptargetModel
+Type: [Array](http://api.jquery.com/Types/#Array)  
+Holds the array that is specified by the `ng-model` attribute of the [`droptarget`](#droptarget) ui-sortable element.
+
+### index
+Type: [Integer](http://api.jquery.com/Types/#Integer)  
+Holds the original index of the item dragged.
+
+### model
+Type: [Object](http://api.jquery.com/Types/#Object)  
+Holds the JavaScript object that is used as the model of the dragged item, as specified by the ng-repeat of the [`source`](#source) ui-sortable element and the item's [`index`](#index).
+
+### moved
+Type: [Object](http://api.jquery.com/Types/#Object)/`undefined`  
+Holds the model of the dragged item only when a sorting happens between two connected ui-sortable elements.  
+In other words: `'moved' in ui.item.sortable` will return false only when a sorting is withing the same ui-sortable element ([`source`](#source) equals to the [`droptarget`](#droptarget)).
+
+### received
+Type: [Boolean](http://api.jquery.com/Types/#Boolean)  
+When sorting between two connected sortables, it will be set to true inside the `update` callback of the [`droptarget`](#droptarget).
+
+### source
+Type: [jQuery](http://api.jquery.com/Types/#jQuery)  
+Holds the ui-sortable element that the dragged item originated from.
+
+### sourceModel
+Type: [Array](http://api.jquery.com/Types/#Array)  
+Holds the array that is specified by the `ng-model` of the [`source`](#source) ui-sortable element.
+
+
+## Methods
+
+### cancel[()](http://api.jquery.com/Types/#Function)
+Returns: Nothing  
+Can be called inside the `update` callback, in order to prevent/revert a sorting.  
+Should be used instead of the [jquery-ui-sortable cancel()](http://api.jqueryui.com/sortable/#method-cancel) method.
+
+### isCanceled[()](http://api.jquery.com/Types/#Function)
+Returns: [Boolean](http://api.jquery.com/Types/#Boolean)  
+Returns whether the current sorting is marked as canceled, by an earlier call to [`ui.item.sortable.cancel()`](#cancel).
+
+### isCustomHelperUsed[()](http://api.jquery.com/Types/#Function)
+Returns: [Boolean](http://api.jquery.com/Types/#Boolean)  
+Returns whether the [`helper`](http://api.jqueryui.com/sortable/#option-helper) element used for the current sorting, is one of the original ui-sortable list elements.

README.md

@@ -46,7 +46,8 @@ Apply the directive to your form elements:
 
 ### Options
 
-All the [jQueryUI Sortable options](http://api.jqueryui.com/sortable/) can be passed through the directive.
+All the [jQueryUI Sortable options](http://api.jqueryui.com/sortable/) can be passed through the directive.  
+Additionally, the `ui` argument of the available callbacks gets enriched with some extra properties as specified to the [API.md file](API.md#uiitemsortable-api-documentation).
 
 
 ```js