add doc for images -viz

docs/sources/commandline/command/images.rst

@@ -10,3 +10,13 @@
 
       -a=false: show all images
       -q=false: only show numeric IDs
+      -viz=false: output in graphviz format
+
+Displaying images visually
+--------------------------
+
+::
+
+    docker images -viz | dot -Tpng -o docker.png
+
+.. image:: images/docker_images.gif