[documentation] Theme developers guide overhaul

[Dale McGladdery]

> > > As you can see I haven't thought it through yet. This is were you
> > > come in. Any ideas?
> > > I'm also looking for people willing to work on this. It's fairly
> > > large, and I'm not sure whether I can do it all by myself.
> >
> > Get an outline and we can start helping to fill it out.
> >
> > Kieran
>
> Pretty much what I said in IRC chat ;)
>
> -sp

Add my name to the helpers list.
(Having a thought out outline as a starting point is awesome in so many ways)