Create code review guidelines

CODEOWNERS

@@ -0,0 +1,11 @@
+# Global owners.
+*           @carbon-language/admin-team
+
+# The core team owns the core language and project documentation.
+/docs/      @carbon-language/core-team
+
+# The review managers own updates to the proposals.
+/proposals/ @carbon-language/review-managers
+
+# This directory contains infrastructure and support scripts
+/src/       @carbon-language/infra-team