Add a big notice header to the github issue template header
Trying to prevent usage questions in Github issue, this change adds a
multi-line notice header directing people to the mailing list.
-[NOTICE]: <> (If your question is around usage and not a bug in Prometheus please use: https://groups.google.com/forum/#!forum/prometheus-users)
+[NOTICE]: <> ( )
+[NOTICE]: <> ( Please do *NOT* ask usage questions in Github issues. )
+[NOTICE]: <> ( )
+[NOTICE]: <> ( If your issue is not a feature request or bug report use: )
+[NOTICE]: <> ( https://groups.google.com/forum/#!forum/prometheus-users. If )
+[NOTICE]: <> ( you are unsure whether you hit a bug, search and ask in the )
+[NOTICE]: <> ( mailing list first. )
+[NOTICE]: <> ( )
+[NOTICE]: <> ( You can read more at: https://prometheus.io/community/ )
+[NOTICE]: <> ( )
**What did you do?**
