Update CHANGELOG.md

Author: AdinAck

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
+- New `serial_echo_irq` example to showcase an interrupt driven serial echo
+
+### Added
+
 - PWM complementary output capability for TIM1 with new example to demonstrate
 
 ### Changed