A question template (#4243)

We get many questions (which is fine), but those are neither a bug, nor
an enhancement. Just add generic question as an option.
Signed-off-by: Miek Gieben <miek@miek.nl>

.github/ISSUE_TEMPLATE/question.md

+---
+name: Question
+about: A question related to CoreDNS
+labels: question
+
+---
+<!-- Please only use this template for submitting a generic question -->