Main app update_schedule override by Default

[rohel01]

Hello,

I am using Bevy to develop a small embedded SW application.
Since I want full control, I am currently setting my App from scratch using App::empty().

Looking at app.rs, I see the following code in App default implementation.

    fn default() -> Self {
        let mut app = App::empty();
        app.sub_apps.main.update_schedule = Some(Main.intern());

Only the Main and Render applications currently set their update_schedule explicitly in Bevy's code base. However, this step seems required for any custom App. Should we better document this? OR maybe we could update the App creation API so it is no longer possible to forget this step?

What do you think?