add note about getting cdebootstrap package from snapshot.debian.org

Since we're using the most recent version, whenever it is updated, the
previous version disappears from the main archives.  Luckily, it can still
be found in snapshot.debian.org
download cdebootstrap from snapshot.debian.org in case the version gets bumped

snapsnot.debian.org keeps archived versions of old packages, while the
mirrors do not.  So if 0.6.5 gets released, then 0.6.4 will disappear from
the mirrors, but still be available on snapshot.debian.org