Fix typo in kubernetes/README.md and remove DEV-README.md as it is stale
and information on the website is more up to date.
Remove large sections of text in plugin.md; just talk about how to
structure your plugin and docs.
* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
