Document the hooks in d-i?

[Petter Reinholdtsen <pere@hungry.com>]

I believe we should make a document listing all the available hooks in
debian-installer, and their usage.  Here is a start with the hooks I
am aware of:

  /etc/rcS.d/S[0-9][0-9]*

    If this is an executable program, it is executed when d-i is
    booting, before the main menu is presented.

 /cdrom/.disk/udeb_include
 /cdrom/.disk/udeb_exclude

    List of packages (udebs) to install or not install in into the d-i
    ramdisk.

 /etc/autopartkit.d/*.table

    Partition table instructions for autopartkit.  The selection is
    done using debconf.

 /cdrom/.disk/base_include
 /cdrom/.disk/base_exclude

    The content of these files are taken as lists of packages to
    install or not into the target area.

 /usr/lib/prebaseconfig.d/*

    The files in this directory are executed in sequence at the end of
    the installation.

Are there more hooks?  Do we need more?