Document that `smtp-relay.google.com` requires a HELLO hostname

Document that smtp-relay.google.com requires a HELLO hostname: Is your feature request related to a problem? Please describe.
I recently spent quite a bit of time trying to figure out why listmonk was failing to send emails from smtp-relay.google.com, but was doing just fine with smtp.google.com. It turns out that the SMTP relay specifically requires the HELLO hostname.