In addition to the marker idea (which is first prize), you can add small notes by creating a Text fader, and then typing into any one of the text fields (when you double-click it).
I use this to label things and add reminders. I usually delete the fader's name but add lots of spaces, otherwise it's almost impossible to grab the thing afterwards to re-position it.
You are limited by the number of characters per text field though. If you want to add complex notes, you can make several entries. You can also add colour.