make the libregraphicsmeeting mailing list subscribers only

[aoloe]

currently we get several spam mails per day which get caught in the filter.

this is rather annoying.

i propose that we make the list refuse mails from people who have not joined the list and send an automatic mail asking them to join the list if they want to post.

on a different ML, we have the following message:

---

Hi!

Posting to the LGM list is restricted to list members.
Not-subscribed addresses are automatically rejected.

If you are not a list member, but would like to get in touch with us
use one of the options listed on our contact page

http://libregraphicsmeeting.org/lgm/contact

or register for this mailing list before sending again your message:

http://lists.freedesktop.org/mailman/listinfo/libre-graphics-meeting

If you are list member: We kindly ask you to send the e-mail again
with your subscribed e-mail address. You can also add a second e-mail
address to the mailinglist, if you often send e-mails from different
e-mail addresses.

Best regards
Ale Rimoldi