Require CODEOWNERS and show (un)covered files

[at15]

This is a feature request

GitHub introduced code owners in July 2017

This new feature automatically requests reviews from the code owners when a pull request changes any owned files

With protected branches enabled, a code owner for each owned file has to leave a review before anyone can merge a pull request to that branch

It would be good if

• maintainer generates an empty CODEOWNERS file or warn if a existing repo doesn't have one (I don't know if maintainer has a health check like feature)
• show the coverage of CODEOWNERS, i.e. some files are not covered

I think k8s already have something similar long time ago, i.e. api/OWNERS. And it's similar to pullapprove in some sense