Add a comment on the HomeScreen

It took me some time to figure out why I had written it this way.

lib/screens/home_screen.dart

@@ -25,6 +25,9 @@ class _HomeScreenState extends State<HomeScreen> {
     Future.delayed(Duration.zero, _initFolder);
   }
 
+  // This is nto done inside build as we want to avoid rebuilding the
+  // FlattenedNotesFolder as much as possible. It's very expensive, since
+  // it sorts all the notes.
   void _initFolder() async {
     if (!mounted) return;