Add README.

Author: Dirbaio

README.md

@@ -0,0 +1,28 @@
+[![crates.io](https://img.shields.io/crates/d/nrf-usbd.svg)](https://crates.io/crates/nrf-usbd)
+[![crates.io](https://img.shields.io/crates/v/nrf-usbd.svg)](https://crates.io/crates/nrf-usbd)
+![Build Status](https://github.com/nrf-rs/nrf-usbd/workflows/CI/badge.svg)
+
+# `nrf-usbd`
+
+> [usb-device](https://github.com/mvirkkunen/usb-device) implementation for Nordic Semiconductor nRF
+microcontrollers.
+
+## Supported microcontrollers
+
+* `nrf52840`
+* `nrf52833`
+* `nrf52820`
+* `nrf5340`, maybe?
+
+## Usage
+
+This driver is relatively low-level, and is intended for use through a HAL library.
+Such HAL library should implement `UsbPeripheral` for the corresponding USB peripheral object.
+This trait declares all the peripheral properties that may vary from one device family to the other.
+
+## Examples
+
+See the [`nrf-hal`](https://github.com/nrf-rs/nrf-hal) for the reference HAL implementation.
+
+See the [`example`](./example) directory for an example on how to use it standalone without a HAL. This is
+discouraged, the recommended usage is through `nrf-hal`.