package: add comment about ipkg/opkg/rpm

Too often the question is raised, that ipkg, opkg and rpm do not work,
and users complain they can not install packages.

Even though we do have a clear and clearly explained section in our
manual, people do not read it (when will users read manuals? sigh...).

So, add a big fat comment about ipkg/opkg/rpm, that Buildroot does not
generate binary packages and does not provide any package database for
any of those package manager.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>