Added readme documentation to mobileMiddleware.

editor/utils/mobile/README.md

@@ -0,0 +1,15 @@
+mobileMiddleware
+===========
+
+`mobileMiddleware` is a very simple [redux middleware](https://redux.js.org/docs/advanced/Middleware.html) that sets the isSidebarOpened flag to false on REDUX_REHYDRATE payloads. 
+This useful to make isSidebarOpened false on mobile even if the value that was saved to local storage was true.
+The middleware just needs to be added to the enhancers list:
+
+```js
+	const enhancers = [
+		...
+		applyMiddleware( mobileMiddleware ),
+	];
+	...
+	const store = createStore( reducer, flowRight( enhancers ) );
+```