LilyPond Architecture explanation

[Carl D. Sorensen]

I've added a brief description of the LilyPond Architecture (for want of a
better word -- suggestions welcome) to the Contributor's guide.  It's based
on what I read in Erik Sandberg's thesis.

I think it's correct, but I'm not positive.  Could I have those of you who
understand things better than me review the material and make corrections or
suggestions?

The explanation in question is at

<http://kainhofer.com/~lilypond/Documentation/devel/contrib-guide/Overview-o
f-LilyPond-architecture.html#Overview-of-LilyPond-architecture>

(Section 7.1 of the CG on kainhofer)


Thanks,

Carl