Add licensing notice

[dtgay]

I do believe you got these sounds from FreeSounds.org, whose attribution terms require that you give credit to the individual creator of the sounds, and probably mention FreeSounds.org. Make sure attribution is always given as asked for when using others' media.

[msoucy]

Somehow this slipped under my radar - I'll update them. For the record, though, I had modified every sound in the repository at that time, from the buzzer @decause gave me and the XBox achievement noise from somewhere on YouTube...

@decause : source for the original buzzer?