Add a link to previous versions of the Handbook
(http://docs.FreeBSD.org/doc/). This should be an answer to various
headaches when it's time to maintain the correctness of a documentation
for the various branches. The Handbook currently covers the use of
FreeBSD 5.4-RELEASE and FreeBSD 6.0-RELEASE (and 7-CURRENT since now the
changes between branches will be less "important"), so the users of
previous releases should read the versions archived on
http://docs.FreeBSD.org/doc/