A new menuName attribute says "Get the contents of the menu from this outline."
Its value is either the name or URL of an outline. If it's a name, it must be the title of one of the outlines on your Roots page.
If it's a URL, it's up to you where to store it. Your Dropbox public folder is a nice place. Not sure Google Drive works for this, because the URLs of their publicly accessible files aren't really to the files. Or so it seems.
There's also a root-level pref for this. It goes alongside the setting of the template through a similar mechanism. Already exists, in the Appearance tab.