The AUTHORS file:

This file should collect a trace of all the legal paperwork that you have
exchanged with contributors for your particular package. This information is
very useful for registering the copyright of your package. The file might
have an introductory blurb similar to this one:

 Authors of PACKAGE

 The following contributions warranted legal paper exchanges
 with [the Free Software Foundation | Your Name].
 Also see files ChangeLog and THANKS

Then, list who the contributors are and what files they have worked on.
Indicate whether they created the file, or whether they modified it. For
example:

 Random J. Hacker:
   entire files  -> foo1.c , foo2.c , foo3.c
   modifications -> foo4.c , foo5.c