Trailing slash required on -I option on Windows

[Urs Liska]

https://github.com/wbsoft/frescobaldi/issues/1094 reports an issue with 
include paths without trailing slashes.

I checked back with 
http://lilypond.org/doc/v2.19/Documentation/usage/command_002dline-usage 
where such a thing isn't documented. So if LilyPond on Windows requires 
a trailing slash for the -I option we should mention it in the manual.

Urs