magick: add append example (#22791)

Update magick.md

pages/common/magick.md

@@ -21,6 +21,10 @@
 
 `magick {{path/to/input_image.png}} -define jpeg:extent={{512kb}} {{path/to/output_image.jpg}}`
 
+- Vertically/horizontally append images and have the empty space be transparent:
+
+`magick convert -background none {{path/to/image1.png path/to/image2.png ...}} {{-append|+append}} {{path/to/output_image.png}}`
+
 - Create a GIF out of all JPEG images in the current directory:
 
 `magick {{*.jpg}} {{path/to/images.gif}}`