Replace COPYING.Music by license.txt in Sound dir

[zabidenhtf]

Because now it too long, and game and music have same license. So in my opinion we can write just like this.

Copyright 2006 (for "menu.mod", "jeu1.xm", "jeu2.xm") MAF464. This music licensed under GPL license. See COPYING for details

[petitlapin]

we don't even need the license.txt, we can add this text in the README.md directly

[zabidenhtf]

Ok, im make pull request with this small update

[zabidenhtf]

#103

[zabidenhtf]

Oh, im forgot to add MAF464's email into readme

[zabidenhtf]

Done, im finished this update

[zabidenhtf]

#104

Reuploaded

[zabidenhtf]

ff6e33e