Add a CODEOWNERS file (#513)

Author: stmontgomery

.github/CODEOWNERS

@@ -0,0 +1,9 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2025 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+* @stmontgomery @grynspan @jerryjrchen @briancroom