debug-layout: trim README and remove lilypond-implementation.

most of the information was already moved to the Frescobaldi fork wiki,
so i've moved the rest (lilypond-implementation, see this wiki revision:
http://github.com/openlilylib/frescobaldi/wiki/Preview-Mode-Development-Roadmap/ec78d7dd452e2e1da883e1640b85bcd9d44096dd)