See the files in the doc directory for documentation about SYSLINUX:

syslinux.txt        - Usage instructions; manual.
distrib.txt     - For creators of Linux distributions.
pxelinux.txt        - Documentation specific to PXELINUX.
isolinux.txt        - Documentation specific to ISOLINUX.
extlinux.txt        - Documentation specific to EXTLINUX.
menu.txt        - About the menu systems.
usbkey.txt      - About using SYSLINUX on USB keys.
comboot.txt     - About the extension API.
memdisk.txt         - Documentation about MEMDISK.

Also see the files:

NEWS            - List of changes from previous releases.
TODO            - About features planned for future releases.
COPYING         - For the license terms of this software.

SYSLINUX now builds in a Linux environment, using nasm. You need nasm version 2.03 or later (2.07 or later recommended) to build SYSLINUX from source. See http://www.nasm.us/ for information about nasm.

There is now a mailing list for SYSLINUX. See the end of syslinux.txt for details.

SYSLINUX is:

Copyright 1994-2010 H. Peter Anvin et al - All Rights Reserved

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, Inc., 53 Temple Place Ste 330, Boston MA 02111-1307, USA; either version 2 of the License, or (at your option) any later version; incorporated herein by reference.