Update README

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

README.md

@@ -1,8 +1,7 @@
 # fluent-plugin-fluent-package-update-notifier
 
-[Fluentd](https://fluentd.org/) input plugin to do something.
-
-TODO: write description for you plugin.
+[Fluentd](https://fluentd.org/) input plugin to notify with
+update logging when newer `fluent-package` is available.
 
 ## Installation
 
@@ -28,13 +27,41 @@ $ bundle
 
 ## Configuration
 
-You can generate configuration template:
+|parameter|type|description|default|
+|---|---|---|---|
+|lts|bool (optional)|Notify fluent-package updates which depends on channel (e.g. Monitor LTS by default)|`true`|
+|notify_major_upgrade|bool (optional)|Notify whether major upgrade is available or not (e.g. from v5 to v6)|`true`|
+|notify_level|enum (optional)|Specify notification log level when update is available (`info`, `warn`)|`info`|
+|notify_interval|integer (optional)|Notify checking update intervals|`86400`|
+
+## Example
+
+If you want to collect update notification via log stream to specific file, let's configure it with @FLUENT_LOG label:
 
 ```
-$ fluent-plugin-config-format input fluent-package-update-notifier
+<source>
+  @type fluent_package_update_notifier
+</source>
+
+<label @FLUENT_LOG>
+  <match fluent.{warn,info}>
+    @type file
+    path notify-fluent-log
+    <buffer []>
+      flush_at_shutdown
+    </buffer>
+  </match>
+</label>
 ```
 
-You can copy and paste generated documents here.
+When shutting down fluentd, `notify-fluent-log_0.log` or similar file (`notify-fluent-log_N.log`) will be saved.
+
+```
+2025-04-21T12:30:53+09:00       fluent.info     {"pid":134883,"ppid":134869,"worker":0,"message":"starting fluentd worker pid=134883 ppid=134869 worker=0"}
+2025-04-21T12:30:53+09:00       fluent.info     {"message":"fluent-package v5.0.6 is available! See https://github.com/fluent/fluent-package-builder/releases/tag/v5.0.6 in details.\n"}
+2025-04-21T12:30:53+09:00       fluent.info     {"worker":0,"message":"fluentd worker is now running worker=0"}
+2025-04-21T12:31:00+09:00       fluent.info     {"worker":0,"message":"fluentd worker is now stopping worker=0"}
+```
 
 ## Copyright