Add serial examples

src/microbit/examples/display_blocking.rs

@@ -36,7 +36,7 @@ fn main() -> ! {
         let mut gpio = p.GPIO.split();
         let mut delay = Delay::new(p.TIMER0);
 
-        // Display
+        // Configure display pins
         let row1 = gpio.pin13.into_push_pull_output().downgrade();
         let row2 = gpio.pin14.into_push_pull_output().downgrade();
         let row3 = gpio.pin15.into_push_pull_output().downgrade();