(doc help: 5 Running lilypond) Scripts in the packages

[Graham Percival]

We need to update the docs to tell people how to run the extra scripts
(convert-ly, midi2ly, etc) from the packages.  Yes, the README tells 
people,
but they don't always read it, and it makes sense to look for that info
in the manual.

I've got a copy of the OSX binary installer, so I could write docs for 
that; do
the windows and linux packages behave in the same way?

- Graham