chore: add CODEOWNERS (#273)

.github/CODEOWNERS

@@ -0,0 +1,7 @@
+# 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
+
+*     @googleapis/yoshi-ruby