Add links to the mailling list

[Luthaf]

We now have a google group as mailling list: https://groups.google.com/forum/#!forum/lumol-users

One can suscribe to the mailling list with the web interface, by sending a mail to lumol-users+suscribe@googlegroups.com, or by visiting https://groups.google.com/forum/#!forum/lumol-users/join.

We should add this information to the website.