--include requires absolute paths

[Federico Bruni]

Should we say explicitly in the doc that -I or --include option 
requires absolute path(s)?
Paragraph to be modified is in Usage 1.2:

-I, --include=directory

   Add directory to the search path for input files.

   Multiple -I options may be given. The search will start in the 
first defined directory, and if the file to be included is not found 
the search will continue in subsequent directories.


Perhaps a warning like "~ cannot be used, use absolute path" may be 
also helpful.

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$ ~/.local/bin/lilypond --include=~/Documenti/spartiti/ly 
Aggressive-Lady.ly
GNU LilyPond 2.19.43
Processing `Aggressive-Lady.ly'
Parsing...
Aggressive-Lady.ly:2:10: error: cannot find file: `myStyle.ly'
(search path: 
`/home/fede/.local/lilypond/usr/share/lilypond/current/fonts/svg/:/home/fede/.local/lilypond/usr/share/lilypond/current/fonts/type1/:/home/fede/.local/lilypond/usr/share/lilypond/current/fonts/otf/:/home/fede/.local/lilypond/usr/share/lilypond/current/scm:/home/fede/.local/lilypond/usr/share/lilypond/current/ps:/home/fede/.local/lilypond/usr/share/lilypond/current/ly:~/Documenti/spartiti/ly:')
\include
        "myStyle.ly"
Aggressive-Lady.ly:13:19: error: unknown escaped string: `\I'

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