Update readme with personal access token note

Describe behaviour reported in #65.

README.md

@@ -191,6 +191,7 @@ You can configure the environment variables in the workflow file like this:
 - When a pull request is merged by this action, the merge will not trigger other GitHub workflows.
   Similarly, when another GitHub workflow creates a pull request, this action will not be triggered.
   This is because [an action in a workflow run can't trigger a new workflow run](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/events-that-trigger-workflows).
+- When [using a personal access token (PAT) to work around the above limitation](https://help.github.com/en/actions/reference/events-that-trigger-workflows#triggering-new-workflows-using-a-personal-access-token), note that when the user issuing the PAT is an administrator and [branch restrictions do not include administrators](https://help.github.com/en/github/administering-a-repository/enabling-branch-restrictions), pull requests may be merged even if they are not mergeable for non-administrators (see [#65](https://github.com/pascalgn/automerge-action/issues/65)).
 - When a check from a build tools like Jenkins or CircleCI completes, GitHub
   triggers the action workflow, but sometimes the pull request state is still
   pending, blocking the merge. This is [an open issue](https://github.com/pascalgn/automerge-action/issues/7).