Update issue template.
Point users at the community website.
<!--
Please note: GitHub issues should only be used for feature requests and
- bug reports. For general usage/help/discussions, please refer to one of:
-
- - #prometheus on freenode
- - the Prometheus Users list: https://groups.google.com/forum/#!forum/prometheus-users
+ bug reports. For general discussions, please refer to one of the community channels
+ described in https://prometheus.io/community/.
Before filing a bug report, note that running node_exporter in Docker is
not recommended, for the reasons detailed in the README:
