Warn about the limitations of GitHub tokens

[dfandrich]

The action will return

Error: HttpError: Resource not accessible by integration

when triggered on a PR on a forked repository.

[dfandrich]

I just discovered issue #399, so this warning should be updated.

[Samriel]

Hey, @dfandrich! Thanks for your contribution! We decided to go with your approach - adding permissions section in README. However we have updated the text.

In order to add labels to pull requests, the GitHub labeler action requires write permissions on the pull-request. However, when the action runs on a pull request from a forked repository, GitHub only grants read access tokens for pull_request events, at most. If you encounter an Error: HttpError: Resource not accessible by integration, it's likely due to these permission constraints.
To resolve this issue, you can modify the on: section of your workflow to use pull_request_target instead of pull_request (see example above). This change allows the action to have write access, because pull_request_target alters the context of the action and safely grants additional permissions.
Refer to the GitHub token permissions documentation for more details about access levels and event contexts.

Could you please change the text like that?

[dfandrich]

Done.