[theme-reviewers] Change 'add_custom_header' from _must_ include to ...

[Lance Willett]

> Themes must support the the following customization feature:
>
> Navigation Menu - wp_nav_menu()
> * At least one custom menu should be supported, generally the
> top-level navigation.

After feedback and thinking about this more, there are themes that
don't include a navigation menu at all.

So, the requirement isn't that themes must include a Custom Menu --
instead, if a navigation menu is part of the design, it should be a
customizable menu using wp_nav_menu().

-Lance