Right now you can keep notes against an article from Edit section.
While in any list swipe the row of which you want to edit to left. A list of icons will come to assist you with 'edit', 'delete' or 'share'. Tap on the 'edit' icon to proceed.
You can write some comments in the notes section. One you are done, tap on the 'tick' icon at top right corner to save the changes.
From any list double tap on any item to see the assigned note against it.