This page is currently about kube-dns and is a bit outdated.
Introduce the heading `# Customizing kube-dns`.

Introduce a separate section about CoreDNS.

Hey Lubomir,
I coypedited pretty heavily because this workflow is so much easier for docs and because I'm trying to help improve everything touching kubeadm as much as possible.

But there's one outstanding issue wrt headings and intro content: you can't add a heading 1 to a topic to do what you wanted to do. The page title in the front matter is rendered as a heading 1 and everything else has to start at heading 2. (We still need to doc this better in the docs contributing content, I know.)

Instead, I think we need to rewrite the top-of-page intro content to explain better the relationship between kube-dns and CoreDNS. I'm happy to write something, but I thought I'd push this commit first so you can see what I'm doing.

Hope it's all clear -- ping here or on Slack with any questions ~ Jennifer