README: Ditch mailing-list for GNOME Discourse

We got loads of spam on the list so even if there were occasional genuine
mails there, I wouldn't notice them and hence not let them through.
4 jobs for master in 2 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #256233
ubuntu-16.04

00:01:53

passed #256234
ubuntu-17.10

00:01:55

passed #256235
ubuntu-18.04

00:02:44

passed #256236
ubuntu-18.04-no-backend

00:01:28