Warn about the limitations of GitHub tokens

The action will return

  Error: HttpError: Resource not accessible by integration

when triggered on a PR on a forked repository.

README.md

@@ -122,6 +122,13 @@ Various inputs are defined in [`action.yml`](action.yml) to let you configure th
 | `configuration-path` | The path to the label configuration file | `.github/labeler.yml` |
 | `sync-labels` | Whether or not to remove labels when matching files are reverted or no longer changed by the PR | `false`
 
+# Limitations
+
+When an action is run for a pull request on on a forked repository, GitHub
+tokens are only given [at most read
+access](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#permissions-for-the-github_token).
+That means this action will not work on pull requests from third parties.
+
 # Contributions
 
 Contributions are welcome! See the [Contributor's Guide](CONTRIBUTING.md).