Review of NR 1.5

[Trevor Daniels]

Hi Francisco

I finally got around to reviewing NR 1.5 Simultaneous notes, which I believe 
you previously edited.  The main changes I made are:

- add section on Simultaneous expressions to 1.5.1
- clarify Single-staff polyphony in 1.5.2
- various miscellaneous other tweaks

I've just pushed the edited version of simultaneous.itely to git master. 
Could you please check it out, either by looking at the .itely file on 
Savannah or when it appears on Reinhold's server, and let me know if you are 
unhappy with any of the changes.

Trevor