nbd: update documentation and link to mailinglist

Documentation/blockdev/nbd.txt contained some documentation which was
horribly outdated and probably still dates from the original patch that
added NBD support to the kernel.

This patch removes the useless and outdated bits.  The tools on nbd.sf.net
are fully documented in manpages, which is where documentation for the
non-kernel bits should live.

Additionally, add a reference to the MAINTAINERS file for the nbd-general
mailinglist that is used for discussion of the userland tools and the
kernel module already.

Signed-off-by: Wouter Verhelst <w@uter.be>
Cc: Paul Clements <Paul.Clements@steeleye.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>