usb: docs

embassy-usb-driver/README.md

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+# embassy-usb-driver
+
+This crate contains the driver traits for [`embassy-usb`]. HAL/BSP crates can implement these
+traits to add support for using `embassy-usb` for a given chip/platform.
+
+The traits are kept in a separate crate so that breaking changes in the higher-level [`embassy-usb`]
+APIs don't cause a semver-major bump of thsi crate. This allows existing HALs/BSPs to be used
+with the newer `embassy-usb` without needing updates.
+
+If you're writing an application using USB, you should depend on the main [`embassy-usb`] crate
+instead of this one.
+
+[`embassy-usb`]: https://crates.io/crates/embassy-usb
+
+## Interoperability
+
+This crate can run on any executor.
+
+## Minimum supported Rust version (MSRV)
+
+This crate requires nightly Rust, due to using "async fn in trait" support.
+
+## License
+
+This work is licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.
+