0.15 migration: Missing section for 16401 - Headless by features

[rparrett]

This breaking change snuck in between rc.3 and 0.15. Users not using default-features may need to add the bevy_window feature.

bevyengine/bevy#16401

[rparrett]

In the first project where I encountered this, my code was explicitly messing with windows and the error was somewhat clear.

I just hit this in a separate project where it manifested in a pretty strange error originating inside of Bevy. In this other project I had the bevy_winit feature included, but not bevy_window.

Documented bevyengine/bevy#16568, because it seems like bevy_winit should also add bevy_window, perhaps.