new italian translator

[Federico Bruni]

Hi folks

Please add to Documentation/included/authors.itexi a new contributor, 
who recently translated the whole Essay manual in italian:

http://git.savannah.gnu.org/cgit/lilypond.git/commit/?h=translation&id=c72179ccf41124936a43bb7279d92c6cb6266b7b

His name is Tommaso Gordini. He made an excellent work. I think that he 
will continue contributing and will help me with finishing and 
reviewing the Notation Reference.

Cheers
Federico