Titlebar widgeting spec finished

This is a follow-up for this issue from 2 weeks ago. Since then, I've written the code for the spec and documented it, and uploaded it here.

I've never contributed to XDG (or anything) before, so I really need some help with what to do next. Is there anyone I should contact to ask if this can become a specification?

EDIT: Should I just send a pull request?