Added link to Rahix/avr-hal to board support list

Author: jonahbron

README.md

@@ -297,6 +297,10 @@ Crates tailored for specific boards.
 - [`pyportal`](https://crates.io/crates/pyportal) - ![crates.io](https://img.shields.io/crates/v/pyportal.svg)
 - [`trellis_m4`](https://crates.io/crates/trellis_m4) - ![crates.io](https://img.shields.io/crates/v/trellis_m4.svg)
 
+### Arduino
+
+- [`avr-hal`](https://github.com/Rahix/avr-hal) - Board support crate for several AVR-based boards including the Arduino Uno and the Arduino Leonardo
+
 ### Nordic
 
 - [`dwm1001`](https://crates.io/crates/dwm1001) - [Decawave DWM1001-DEV] - ![crates.io](https://img.shields.io/crates/v/dwm1001.svg)