Such a feature already exists. Open up a waypoint or route. Go to the "Notes" tab and you will see "Add File Link" and "Add Web Link"
That I know. You probably didn't understand. It would be nice to have this for a List or a List Folder as well. So you can add file links or web links that have something to do with all the routes, waypoints, picture and other objects in the list. Without adding this information to every waypoint or route seperately.
@DaanVenlo
That's an interesting idea. Can you tell me more about how/why you'd like that feature? It sounds like something that might come in handy when sharing lists.