Expand subscribe_mailinglist template

The template needs more human-readable information. Now that the mailing list is
handled by Google Groups, there is no way to subscribe by simply sending an email.
I have configured the production server to send these emails to help@datadryad.org,
and they will be processed manually.
Correct typo
Merge pull request #822 from ryscher/mailinglist-subscription-template

Expand subscribe_mailinglist template