Implement CODEOWNERS

[pllim]

Create a new .github/CODEOWNERS (https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners) as @jonathansick suggested in astropy/astropy.github.com#525 (comment)

Also see:

• Implement CODEOWNERS astropy-tutorials#580