Extend README for NetBSD.
Extend README for NetBSD.

Add NetBSD to the list of systems on which bup is known to work.  Give
hints for bup usage on NetBSD, including the location of the fuse
bindings and the pkgsrc entry.  Caution about incorrect cycle
detection on fuse mounts.  Add pkgsrc URLs.

Signed-off-by: Greg Troxel <4eec6eaa46bbe8669dbb7b048f657d9b10b802ca@lexort.com>
Reviewed-by: Gabriel Filion <8df7619ec42e51274a2f3ff6cd1afa366f63857b@gmail.com>
Reviewed-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

pombredanne/bup,jbaber/bup,gevaerts/bup,jbaber/bup,mhoeher/bup,ToxicFrog/bup,ToxicFrog/bup,tjanez/bup,gevaerts/bup,mhoeher/bup,apenwarr/bup,pombredanne/bup,tjanez/bup,pombredanne/bup,tjanez/bup,mhoeher/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,pombredanne/bup,gevaerts/bup,gevaerts/bup,apenwarr/bup,gevaerts/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,jbaber/bup,apenwarr/bup,ToxicFrog/bup,ToxicFrog/bup,jbaber/bup,mhoeher/bup,tjanez/bup,tjanez/bup,apenwarr/bup,jbaber/bup,ToxicFrog/bup