You can record little notes or reminders for yourself in the 'Notes' tab of the Quota Panel. Click the '+' in the top right corner of the panel to add a note. A field for the note will appear, where you can write the note and press 'Ok'.

To edit a note, click the Pencil button on the top right corner of the note, and make the desired adjustments. To delete a note, press the Trashcan button, and click 'Yes' on the ensuing confirmation.