README: update mailing list

Update the top-level README.md file to refer to the new mailing list.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/README.md b/README.md
index 156c87c..6c9a0dd 100644
--- a/README.md
+++ b/README.md
@@ -58,7 +58,7 @@
 Developers welcome!
 
 * Our developer mailing list:
-  http://lists.runtime.co/mailman/listinfo/dev-mcuboot_lists.runtime.co
+  https://groups.io/g/mcuboot
 * Our Slack channel: https://mcuboot.slack.com/ <br />
   Get your invite [here!](https://join.slack.com/t/mcuboot/shared_invite/MjE2NDcwMTQ2MTYyLTE1MDA4MTIzNTAtYzgyZTU0NjFkMg)
 * Our IRC channel: http://irc.freenode.net, #mcuboot