Update README.md

Author: aisin

README.md

@@ -123,7 +123,7 @@ More usages or getting start you can refer to these clearly [examples](https://g
 
 | **             Prop             ** | **Type**  | **Description**                                                                                                                              |
 |------------------|---------------|---------------------------------------------------------------------------------------------------------------------------------------------------|
-| `data-key`       | String        | The unique key get from `data-sources` in each data object, its value **must be unique** in `data-sources`, it is used for identifying item size. |
+| `data-key`       | String\|Function        | The unique key get from `data-sources` in each data object, or generated by the attributes in each data object of `data-sources`. Its value **must be unique** in `data-sources`, it is used for identifying item size. |
 | `data-sources`   | Array[Object] | The source array built for list, each array data must be an object and has an unique key for `data-key` property.                                 |
 | `data-component` | Component     | The render item component created / declared by vue, and it will use the data object in `datas-sources` as render prop and named: `source`.       |