Add reference to links in DNS doc
Add reference to links in DNS doc

We refer to links a bunch in the doc, it's useful for the reader to have
context on it.

[#146393219](https://www.pivotaltracker.com/story/show/146393219)

Co-authored-by: Florian Nachtigall <da0e822b67ef154a118f514785d782d9f0817746@sap.com>

cloudfoundry/docs-bosh