This feature allows you to set alternatives [1].
For example, like this:

   @$(call add,ALTERNATIVES,/usr/bin/xvt:/usr/bin/xterm)

Also, in the config itself, targets for setting alternatives may already
be predefined. For example, use/alternatives/xvt/% allows you to specify
arbitrary alternatives for xvt:
use/alternatives/xvt/xterm, use/alternatives/xvt/mate-terminal, etc.
However, an alternative must be available.

1. https://www.altlinux.org/Alternatives

The @META suffix is used to expand the metapackage into a list.
apt asks for the dependencies of such a package and adds them to the
package list after this metapackage.

The BRANCH parameter indicates that the target is a starterkit and
not a regular build for Sisyphus.

The new archdep part has been initially included into
"The Basics" chapter which it doesn't really belong to,
let's move it into Addendum.

NB: I'd better try building a package and skimming over
    at least the index upon having modified docs next time.

This is an initial implementation of architecture dependent
contents handling for package lists more or less in the vein
of mkimage-profiles-desktop's one *but* using suffix part to
filter words in or out *not* prefix part to replace it with
a comment marker (thus filtering out lines).

The syntax should be pretty obvious:

  a b@i586 c@x86_64

will get "a b" given ARCH=i586 and "a c" given ARCH=x86_64;
please see doc/archdep.txt for a more elaborate description
and a conversion script.

The goals listed are pretty important to have them ordered
by priority; collaboration is definitely more important
than dynamic range of release managers' experience.

icewm.iso is an installer needing further hassle to try;
rescue.iso is immediately useful OTOH.

"Goals" paragraph ended up with all of list items on a single line;
a colon has been missing in another one.

It got missed somehow that it's a bullet list indeed...

These have been proofread somewhat to correspond
to the current state of affairs; a missing one
was added for fonts feature.

Docs should live happily together, after all,
and toplevel should be more or less uncluttered :)

Adjusted/rewrote as appropriate as well.