* plugin/metadata: finish documentation

Finish the README.md, add corner case in the IsLabel test and reword
some code comments slightly.

Generate the man-pages and add man/coredns-metadata.7 as well.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix test
Signed-off-by: Miek Gieben <miek@miek.nl>