examples: Add comment to explain about files

examples/contextlost/main.go

@@ -71,6 +71,15 @@ func update(screen *ebiten.Image) error {
 }
 
 func main() {
+	// Decode image from a byte slice instead of a file so that
+	// this example works in any working directory.
+	// If you want to use a file, there are some options:
+	// 1) Use os.Open and pass the file to the image decoder.
+	//    This is a very regular way, but doesn't work on browsers.
+	// 2) Use ebitenutil.OpenFile and pass the file to the image decoder.
+	//    This works even on browsers.
+	// 3) Use ebitenutil.NewImageFromFile to create an ebiten.Image directly from a file.
+	//    This also works on browsers.
 	img, _, err := image.Decode(bytes.NewReader(images.Gophers_jpg))
 	if err != nil {
 		log.Fatal(err)