Who reviews this code: use CODEOWNERS to assign reviewers by directory

When a directory changes, the people responsible for that path can be added as reviewers automatically. AngusGit reads the CODEOWNERS file in the repository. After you turn the switch on, a new pull request adds the matching people as reviewers. This article shows how to write the file, where the switch is, and how it combines with the default reviewers, using xlliu/test-pr-example #9 as the check.