If you don't have an SMTP server already, just setup a nim-lang-dev list at google groups to get the list going. Their web interface is terrible, but all you need is the list.
I can offer a "mlmmj" backed list as well if you need/want to keep the nim-lang.org domain, it's literally a 2seconds process.
If you want to keep a web-based interface, Phorum has already a mailing list gateway (nginx forums are ml-backed AFAIK). There are numerous others to choose from without implemeting a 2way gateway again.
That being said, I would recommend just subscribing the list to gmane.org and point users to their threaded view. As a teaser, please see:
http://news.gmane.org/gmane.comp.lang.rust.devel/cutoff=11455
It's a "no frills" interface, but allows full text searching and posting without subscription as well.