fix(README): use the new etcd-dev mailing list

We moved to google groups, update the URLs

README.md

@@ -90,7 +90,7 @@ You have successfully started an etcd on a single machine and written a key to t
 
 ## Contact
 
-- Mailing list: [etcd-dev](http://coreos.com/lists/etcd-dev/)
+- Mailing list: [etcd-dev](https://groups.google.com/forum/?hl=en#!forum/etcd-dev)
 - IRC: #[coreos](irc://irc.freenode.org:6667/#coreos) oon freenode.org
 - Planning/Roadmap: [milestones](https://github.com/coreos/etcd/issues/milestones)
 - Bugs: [issues](https://github.com/coreos/etcd/issues)