docs(datepicker): add docs for new two properties onOpenModal and onCloseModal

Author: feyy

README.md

@@ -79,6 +79,8 @@ You can check [index.js](https://github.com/xgfe/react-native-datepicker/blob/ma
 | is24Hour | - | `boolean` | Set the TimePicker is24Hour flag. The default value depend on `format`. Only work in Android |
 | placeholder | '' | `string` | The placeholder show when this.props.date is falsy |
 | onDateChange | - | `function` | This is called when the user confirm the picked date or time in the UI. The first and only argument is a date or time string representing the new date and time formatted by [moment.js](http://momentjs.com/) with the given format property. |
+| onOpenModal | - | `function` | This is called when the DatePicker Modal open. |
+| onCloseModal | - | `function` | This is called when the DatePicker Modal close |
 | modalOnResponderTerminationRequest | - | `function` | Set the callback for React Native's [Gesture Responder System](https://facebook.github.io/react-native/docs/gesture-responder-system.html#responder-lifecycle)'s call to `onResponderTerminationRequest`. By default this will reject a termination request, but can be overidden in case the View under the Modal is implementing custom gesture responders, and you wish for those to be overidden in certain cases.  |
 
 ### Property `customStyles` available keys