updated docs

README.md

@@ -252,6 +252,10 @@ export default connect(store => ({
 })(Todos);
 ```
 
+#### Callback on `store.updateStore`
+
+`store.updateStore` has a second optional parameter which should be a `function` that will be run as callback of `setState`. This callback will receive the store's update state as it's only parameter. Please see [react's docs](https://reactjs.org/docs/react-component.html#setstate) about `setState`.
+
 ## How to handle async actions?
 
 The package itself does not care how you handle this, you can use `async/await` if you like or stick to the chained `.then` of promises. But don't use generator functions as the store package was not equipped with it and supporting it is not an option because it would defeat the whole purpose of this library.