message: Finish `iced_native` usage section
diff: //! shells (like [`iced_winit`]) can use this trait to stay renderer-agnostic.
  //!
  //! # Usage
  -//! Check out the [`UserInterface`] type to learn how to wire everything up!
  +//! The strategy to use this crate depends on your particular use case. If you
  +//! want to:
  +//! - Implement a custom shell or integrate it in your own system, you should
  +//! check out the [`UserInterface`] type.
  +//! - Build a new renderer, see the [renderer] module.
  +//! - Build a custom widget, start at the [`Widget`] trait.
  //!
  //! [`iced_core`]: https://github.com/hecrj/iced/tree/master/core
  //! [`iced_winit`]: https://github.com/hecrj/iced/tree/master/winit
  //! [`Widget`]: widget/trait.Widget.html
  //! [`Windowed`]: renderer/trait.Windowed.html
  //! [`UserInterface`]: struct.UserInterface.html
  +//! [renderer]: renderer/index.html
  #![deny(missing_docs)]
  #![deny(missing_debug_implementations)]
  #![deny(unused_results)]
  