Right now we have a contest going on for civilizations to be put into a supplement. If you'd rather not have yours considered for inclusion, please put that information somewhere in your post. Civs designed for that contest should go into the folder under "Challenges & Contests"
Societies based on other people's work (e.g. one based on Ian M. Banks' Culture) are definitely welcome here, but are disqualified from the contest thanks to intellectual property rights.