This plugin (Icy Tool) allows the user to create and edit annotations. Annotations are blocks of text that the user can put anywhere in the sequence. Run the plugin to get the GUI with all the features. Once a note is added to a sequence, left click on it to edit this one. If you closed your GUI and you have some notes on your sequence, right click and select "edit" to display the GUI.
As of now, the annotations are saved when the sequence is closed, but you have to run the plugin at least once to be able to see all the annotations in the image.