README: update mailing list
Update the top-level README.md file to refer to the new mailing list.
@@ -58,7 +58,7 @@ For more information in the source, here are some pointers:
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
