chore: create CODEOWNERS (#251)

Author: kweinmeister

CODEOWNERS

@@ -0,0 +1,10 @@
+# Code owners file.
+# This file controls who is tagged for review for any given pull request.
+#
+# For syntax help see:
+# https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners#codeowners-syntax
+
+
+# The php-admins team is the default owner for anything not
+# explicitly taken by someone else.
+* @GoogleCloudPlatform/php-admins