Update file week12.md

docs/assignments/week12.md

@@ -258,6 +258,10 @@ But an hour later it decides to do some work and this is the neopixel show all t
 
 I want my crystals to flash white for 2 seconds and then stops for 2 seconds.
 
+Here is a very useful video on how to do that:
+
+<iframe width="1000" height="400" src="https://www.youtube.com/embed/z1IGAbIFGnk?si=tfGQKLB7yPL2Uyc9" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
+
 And here's the code:
 
     #include <Adafruit_NeoPixel.h>