Add CODEOWNERS file for automated reviewer assignment (#207)

This PR adds a CODEOWNERS file to automatically assign the appropriate
maintainer team for pull request reviews, addressing the manual reviewer
assignment issues outlined in
flatcar/Flatcar#1791.

Author: John15321

CODEOWNERS

@@ -0,0 +1,5 @@
+# CODEOWNERS file for flatcar-linux-update-operator
+# This file defines who is responsible for code review
+# See: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+* @flatcar/flatcar-maintainers