Add anguillanneuf as codeowner for Pub/Sub (GoogleCloudPlatform#2607)

Author: anguillanneuf

.github/CODEOWNERS

@@ -21,4 +21,7 @@ iot/*                     @gguuss
 notebooks/*               @alixhami
 
 # Torry Yang is the primary maintainer of the Tables samples.
-/tables/               @sirtorry
+/tables/                  @sirtorry
+
+# Tianzi Cai is the primary maintainer of the Pub/Sub samples.
+/pubsub/                  @anguillanneuf