She is offering to outfit a horse with armor at no charge to anyone who presents her with a note like the one that I've found. I've discovered a notice from Snak gra-Bura of the Chestnut Handy stables.
Update: Upon installation of the mod and entrance into the game:.When reading the document, it says 'I'm a set of plans. They count as quest items and cannot be dropped, although they are not used in any quest and become permanently stuck in the inventory. Occasionally, glitched or unfinished notes in certain caves labelled ' Horse Armor Plans' can be encountered. If a horse is not owned before going to Snak gra-Bura, she will give a free one with armor. Subsequent horse armor may be purchased for 500. She will give a free set of either Elven Horse Armor or Steel Horse Armor. Once the Hero of Kvatch receives the note, simply speak to Snak gra-Bura at the Chestnut Handy Stables outside the Imperial City.