Add docs for out.params.label.

Author: nat-henderson

README.md

@@ -182,6 +182,8 @@ Set the status message for `concourse-ci` context on specified pull request.
 
 * `merge.commit_msg`: *Optional.* Used with `merge` to set the commit message for the merge. Specify a file path to the merge commit message.
 
+* `label`: *Optional.* A label to add to the pull request.
+
 ## Example pipeline
 
 Please see this repo's [pipeline](https://github.com/jtarchie/pullrequest-resource/blob/master/.concourse.yml) for a perfect example.