Merge pull request #4336 from marmelab/fix-custom-layout-notification

Fix custom layout documentation doesn't mention the importance of the <Notification> component

docs/Theming.md

@@ -605,6 +605,8 @@ MyLayout.propTypes = {
 export default MyLayout;
 ```
 
+**Tip**: Don't forget to render a `<Notification>` component in your custom layout, otherwise the undoable updates will never be sent to the server. That's because part of the "undo" logic of react-admin lies in the `<Notification>` component.  
+
 ## Customizing the AppBar Content
 
 By default, the react-admin `<AppBar>` component displays the page title. You can override this default by passing children to `<AppBar>` - they will replace the default title. And if you still want to include the page title, make sure you include an element with id `react-admin-title` in the top bar (this uses [React Portals](https://reactjs.org/docs/portals.html)).