Correct repository URL; note mailing list subscription URL

The repository URL noted a development site, and should have been
changed on migration to the working group's site.

The mailing list URL was provided to The SleuthKit's user and developer
lists.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Signed-off-by: Douglas White <douglas.white@nist.gov>
Signed-off-by: Mary Laamanen <mary.laamanen@nist.gov>
Signed-off-by: John Tebbutt <john.tebbutt@nist.gov>