docs: Add notes about pull_request on forks

README.md

@@ -78,6 +78,8 @@ jobs:
           # config_file: .github/label-commenter-config.yml
 ```
 
+For the `pull_request` event from forks, we need to use a personal access token instead of `GITHUB_TOKEN`. The generated `GITHUB_TOKEN` on a fork has no write permission. See the official documentation for more details. [Authenticating with the GITHUB_TOKEN - GitHub Help](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token)
+
 ### Action Setting
 
 ```yaml