[doc-request] fret diagrams

[Juergen Reuter]

Hi!

In input/test/fret-diagram.ly, we have a nice fret diagram, but I can not 
find any section in the manual about it (or did I miss something?).  Since 
the NEWS file advertises fret diagrams for the upcoming version of lily, 
it would be nice to have a short section about frets in the user manual, 
maybe next to section 5.13 (tablatures).  Is there somebody over here who 
knows enough about guitars to write such a section?

Greetings,
Jürgen