update irc link

according to an email to the dev mailing list, the official irc channel has moved. updated link here.

CONTRIBUTING.md

@@ -16,7 +16,7 @@ more information.
 
 Before you spend a lot of time working on a new feature, it's always best to
 discuss your proposed changes with us first.  The best place to do that is in
-our IRC channel on **irc.freenode.net**, channel **#darktable** or the
+our IRC channel on **ircs://irc.oftc.net:6697**, channel **#darktable** or the
 development mailing list, [see here for more
 information](https://www.darktable.org/contact/).  This will dramatically
 improve your chances of having your code merged, especially if we think you'll