Add mention of atsamd51 implementation to README

Author: Tom

README.md

@@ -23,6 +23,9 @@ Hardware driver crates
 * [stm32-usbd](https://github.com/stm32-rs/stm32-usbd) - device-driver implementation for multiple STM32 microcontroller families.
   Examples can be found in [stm32-usbd-examples](https://github.com/stm32-rs/stm32-usbd-examples).
 
+* [atsamd](https://github.com/atsamd-rs/atsamd) - device-driver implementation for samd51 microcontrollers. An example for the
+  itsybitsy_m4 board from Adafruit can be found [here](https://github.com/atsamd-rs/atsamd/blob/master/boards/itsybitsy_m4/examples/usb_serial.rs).
+
 Class crates
 ------------