README: update for current state of FreeBSD support

The FreeBSD project hasn't supported releases prior to 8.x for a long
time now, and OpenAFS is neither built nor tested regularly on
anything that old.  Dedocument support for these releases in
preparation for later removing configuration support.

Change-Id: I42e78291cfed91e20b3414576cf885a11b7f341b
Reviewed-on: http://gerrit.openafs.org/11437
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>