Next to the set same on the "Inserts/Related page should be a little (edit) link. If it's now there, it means you don't have the permissions to do so yet (that will come with more participation and visting the Permission link under Account below). If that's the case, and you cannot update the Release Notes, just go to the checklist page for the set you wish to add the note to, and click on the "See any inaccuracies?" link, and report it there. Someone else who has the proper permissions will enter the text (which, no matter who does it, will go to Admin for final approval).
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).