Adjust mailing list signup process

That Mailman URL turned out to be not accessible off of NIST's campus.
Fortunately, the by-email list interface works as expected.

Reported-by: Eoghan Casey <eoghan@disclosedigital.com>
Tested-by: Mary Laamanen <mary.laamanen@nist.gov>
Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>