convert: add vertical append example (#3581)

pages/common/convert.md

@@ -19,6 +19,10 @@
 
 `convert {{image1.png}} {{image2.png}} {{image3.png}} +append {{image123.png}}`
 
+- Vertically append images:
+
+`convert {{image1.png}} {{image2.png}} {{image3.png}} -append {{image123.png}}`
+
 - Create a gif from a series of images with 100ms delay between them:
 
 `convert {{image1.png}} {{image2.png}} {{image3.png}} -delay {{100}} {{animation.gif}}`