add details about the required permissions to the readme

Author: jazzsequence

README.md

@@ -43,4 +43,8 @@ The GitHub token to use for creating a PR. If not specified, the action will use
 A comma-separated list of filenames to check for the "Tested Up To" version. If not specified, the action will use `readme.txt` and `README.md`.
 
 #### `branch`
-The branch to create the PR against. If not specified, the action will use the `main` branch.
+The branch to create the PR against. If not specified, the action will use the `main` branch.
+
+## Permissions
+
+The `write` permissions on `contents` and `pull-requests` are important. They are required for the action to commit the changes back to the repository and open a pull request. The only files affected by the action are files named `readme.txt`, `README.md` or those files matching the pattern (looking for "Tested Up To" in the file) that have been specified in the `filenames` input.