From the rules:
24. Advertisements (including signatures within the post area) in posts aren't allowed unless the post is in a thread you started and is really substantial and useful.
I thought this needed custom permission from Admin:
~did you get permission from theymos to advertise there as you need to get the go-ahead from an admin to do that and it's rarely granted?
I do think hilariousandco's interpretation is better than the one in the official unofficial rules.