add ISSUE_TEMPLATE to route to discourse/slack (#1586)
add ISSUE_TEMPLATE to route to discourse/slack (#1586)

Landing on the New Issue page, it's not clear to users that they should
start by asking questions in discourse and slack.

This change puts instructions directly into the description of a New
Issue.
linkerd/linkerd,linkerd/linkerd,linkerd/linkerd,BuoyantIO/linkerd,BuoyantIO/linkerd