How about .meta? Hopefully we can encourage github to support this, too. Maybe .meta/github/ could be a thing, too.
I personally do not like ".meta", because it is not clear what kind of information it may contain. "Meta" might be anything, documentation, description, website, design papers, everything is meta. It is a bit like using "Information" as a menu entry for a website.
I'd prefer .well-known since there's already an RFC for it, but maybe .dev-extra would be more appropriate?