add mailing list

CONTRIBUTING.md

@@ -40,8 +40,12 @@ Please provide the following details so that your problem can be fixed:
 
 ## Development
 
+### Contact us!
+
 We usually hang out on **irc.freenode.net** in the **#owncloud-news** and **#owncloud-dev** chat room. Just ping [Raydiation](https://github.com/Raydiation/) or [cosenal](https://github.com/cosenal) or write us a mail directly. Mail addresses are listed on our GitHub profiles.
 
+You can also send a mail to the [owncloud-devel mailing list](https://mailman.owncloud.org/mailman/listinfo/devel).
+
 ### Project structure