Preface need update.

[Manolis Kiagias]

Murray Stokely wrote:
> Thanks Manolis.  Maybe we should add a comment to the Makefile so that
> when authors commit new chapters they remember to also update the
> preface summary.  Or alternatively, we could put some kind of one
> sentence chapter description in <chapterinfo> of each chapter and pull
> it out of each file dynamically when building the preface.  I'd
> probably recommend the low tech first suggestion as good enough.
>
>         - Murray
>
>   

Yes, I think the Makefile solution is good enough.
I always read the comments on important files, and hopefully most 
committers do, so it won't go unnoticed.

Cheers,
Manolis