docs: more on imports and snippets (#2423)
docs: more on imports and snippets (#2423)

Add some docs in the manual page. Problem here is that it is only in the
manual page and we don't publish this particular one on coredns.io.

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
chrisohaver/coredns,johnbelamaric/coredns,coredns/coredns,ii/coredns,miekg/coredns,johnbelamaric/coredns,miekg/coredns,yongtang/coredns,ii/coredns