staging: document that the wifi staging drivers a bit better

The Linux wireless developers don't want to hear anything about the
staging wireless drivers, for a wide range of miopic reasons.

The following patch, based on a patch from Johannes Berg, tries to
document this issue a bit better.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>