middleware/kubernetes: Rewrite/expand readme (#764)

* rewrite readme

* remove breaks

* missed a break

* nits

* show options as optional
* add note to pods insecure
* add minimal configuration example.
* add a note about replacing kube-dns in the summary
* move deployment section into summary.

* Update README.md

* replace options sections with bullets

and indent all sub sections to match bullet indentation.
not sure if this will render in github properly - it doesn't in the in-line editor preview.

* fix indentation

Contrary to various on-line guides and editors, github now requires 2 spaces to indent paragraphs with bulleted sections above (not just 1).

* Update README.md

* Update README.md

* Update README.md

* fix label syntax

It's just a single EXPRESSION.  The EXPRESSION itself has its own kubernetes label expression format.

* Update README.md

* Update README.md