Board & Card Games Stack Exchange is a question and answer site for people who like playing board games, designing board games or modifying the rules of existing board games. It only takes a minute to sign up.
http://wiki.dominionstrategy.com/index.php/Promo. This page links to all promo cards, each page quotes the official FAQs. Note that there are no special "rules" associated with the promo cards, in general what you need to know to play them is on the card text. But there are official FAQs/clarifications, like there are for every card. They should have come on a sheet with the cards themselves.