Document how to restrict the action (#51)

Fixes #49

Author: fkorotkov

README.md

@@ -30,3 +30,7 @@ jobs:
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 ```
+
+## Restricting who can call the action
+
+It's possible to use `author_association` field of a comment to restrict who can call the action and skip the rebase for others. Simply add the following expression to the `if` statement in your workflow file: `github.event.comment.author_association == 'MEMBER'`. See [documentation](https://developer.github.com/v4/enum/commentauthorassociation/) for a list of all available values of `author_association`.