add youtube video link

index.html

@@ -174,7 +174,11 @@
       <label for="settings-qr"> Output QR code</label><br>
     </span>
     <br><br>
-    <a class="source-link" href="https://github.com/p2r3/ha.mr">Source code on GitHub</a>
+    <div>
+      <a class="source-link" href="https://github.com/p2r3/ha.mr">Source code on GitHub</a>
+      |
+      <a class="source-link" href="https://www.youtube.com/watch?v=TOr1Vvji6jA">Explanation video</a>
+    </div>
 
   </div>