Document adding new color spaces, ∆E methods, and gamut mapping methods

[facelessuser]

We've mentioned in the docs that you can create your own color spaces and register them (along with ∆E methods and gamut mapping methods) but we've never actually documented how this can be done. This is mainly because we weren't 100% sure if the solution was going to change. I think moving forward, and especially before a 1.0 release, we should document how this can be done.

[facelessuser]

We've got something in that covers this: c51813c. We may improve it in the future, but at least we do cover it now.