Yes. Go to Manage Series and choose the Affiliate and Series you wish to annotate and select the Notes tab. Notes for other organisers can be very helpful to give guidance on specific aspects of what you need to do in organising the event. While they could be tips on how to operate ManageMyMatch in the context of the event is probably more meaningful to give external organisational requirements, expectations or tips.
While you can just enter simple text, ManageMyMatch uses a simple formatting language called MarkDown that allows you to better format and organise your text. When you edit your note, a link is given to a web site that will take you through the basics of how MarkDown works.
Any notes added will become available to organisers via a button (Organiser's notes) in Manage Events. Pressing this will expand the screen to show your notes. You can also choose to edit the notes from here which simply takes you directly to the page as above.