Updated Design: Editor Themes (markdown)

Design:-Editor-Themes.md

@@ -9,6 +9,7 @@ See also [issue #610](https://github.com/node-red/node-red/issues/610)
         header: {
             title: "Node-RED",
             image: "/absolute/path/to/header/image" // or null to remove image
+            url: "http://nodered.org" // optional url to make the header text/image a link to this url
         },
         
         deployButton: {