color management: add intro doc

Hi,

I've taken long enough with this, and while it probably doesn't have everything I had in mind and may seen a bit incomplete, I'd like to get it into your wayland-protocols MR sooner rather than later.

Feel free to squash this in, or keep as a separate commit, or add more to it. Just keep me credited in the commit message and Collabora's copyright. If you add or edit, add your copyright as well.