Explain required attributes (#191)

FabricImage will not be able to use the video element if the width and height attributes are not set. This is not obvious, so adding a comment about it so people won't waste time figuring out why there code does not work.

Author: Tey

src/content/demo/video-element/code.js

@@ -7,6 +7,7 @@ const video2source = document.createElement('source');
 
 const webcamEl = document.createElement('video');
 
+// FabricImage requires the width and height attributes to be set
 video1El.width = 480;
 video1El.height = 360;
 video1El.id = 'video1'
@@ -107,4 +108,4 @@ if (window.location.protocol === 'https:') {
 fabric.util.requestAnimFrame(function render() {
   canvas.renderAll();
   fabric.util.requestAnimFrame(render);
-});
+});