Mention allowbuilddep name="*" feature

<allowbuilddep name="*" /> allows any project depend on binaries
produced by this project.

This addresses the licensing constraints imposed for packages like
the commercial Qt distribution, where sources must not be redistributed
but the resulting binaries are not subject to any restrictions.

Currently, only "*" is supported as a pattern, anything else is
to be implemented in the event of sufficient demand.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>