winit-core: add top-level doc

winit-core/src/lib.rs

@@ -1,3 +1,12 @@
+//! # Core types for Winit
+//!
+//! Platform-agnostic types and traits useful when implementing Winit backends,
+//! or otherwise interfacing with Winit from library code.
+//!
+//! See the [`winit`] crate for the full user-facing API.
+//!
+//! [`winit`]: https://docs.rs/winit
+
 #[macro_use]
 pub mod as_any;
 pub mod cursor;