README.md in fluent-plugin-gcloud-pubsub-custom-0.3.4 vs README.md in fluent-plugin-gcloud-pubsub-custom-0.4.0

- old
+ new

@@ -103,9 +103,12 @@
 </source>
 ```
 
 - `tag` (required)
   - Set tag of messages.
+  - If `tag_key` is specified, `tag` is used as tag when record don't have specified key.
+- `tag_key` (optional)
+  - Set key to be used as tag.
 - `project` (optional)
   - Set your GCP project
   - Running fluentd on GCP, you don't have to specify.
   - You can also use environment variable such as `GCLOUD_PROJECT`.
 - `key` (optional)