Update README.md

serialtimetest/README.md

@@ -48,7 +48,9 @@ Shopbot/0.1 mil steps/10x microstep = 78,740 steps/s
 
 # Raspberry Pi 4
 
-nice -n -20, 10 kHz
+~ same with nice -n -20, 10 kHz
+
+[Preempt-RT real-time kernel?](https://lemariva.com/blog/2019/09/raspberry-pi-4b-preempt-rt-kernel-419y-performance-test)
 
 <img src=Pi4.SDS00005.png width=75%><br>
 <img src=Pi4.SDS00004.png width=75%><br>
@@ -56,3 +58,5 @@ nice -n -20, 10 kHz
 # [timer loops](serialtimetest.c)
 
 # [Web serial](serialtimetest.html)
+
+[API](https://web.dev/serial/)