Added source of truth comment

Author: gkevinzheng

gapic_templates/admin_v2/types/oneof_message.py.j2

@@ -13,6 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+#
+# AUTOGENERATED FILE: DO NOT EDIT. The source of truth for this file is under
+# the gapic_templates directory, not this one.
+
 import collections.abc
 import proto
 

google/cloud/bigtable/admin_v2/types/oneof_message.py

@@ -13,6 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+#
+# AUTOGENERATED FILE: DO NOT EDIT. The source of truth for this file is under
+# the gapic_templates directory, not this one.
+
 import collections.abc
 import proto