Also mention newly-implemented samd21 UsbBus implementation

Author: Tom

README.md

@@ -23,7 +23,7 @@ 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
+* [atsamd](https://github.com/atsamd-rs/atsamd) - device-driver implementation for samd21 & 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