From my reading of the extension page, ReadingLists doesn't provide a web UI for users to access a reading list to add/remove items, correct? It seems ReadingLists implements an API which would need additional code in order to allow users to easily add and remove items from their reading lists? Thanks
Topic on Extension talk:ReadingLists/Flow
That's correct, unfortunately. The relevant task is T275227. There is a browser plugin (Chrome, Firefox, source) but it only lets you add things.