title: How to Post PR comment from jenkinsfile
body: <p>I am running Multibranch pipeline jobs using Github plugin with the option below
  <a href="https://i.stack.imgur.com/OnGNW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OnGNW.png" alt="Discover branches"></a></p>
  
  <p>and periodical polling (5 mins) to look for open pull requests (new feature branches in our case). Open pull request is identified, job is triggered and on successful completion am able to update the status. would like to add a PR status message with URL using</p>
  
  <blockquote>
    <p><code>pullRequest.comment('This is a comment sent from the Pipeline').</code></p>
  </blockquote>
  
  <p>But pipeline job fails with </p>
  
  <blockquote>
    <p>"No such property: pullRequest for class: groovy.lang.Binding".</p>
  </blockquote>
  
  <p>on further inspection, <strong>env.CHANGE_ID</strong> is set to NULL (means it is not a PR build job which is a prerequisite to use pullRequest module). can anyone suggest some idea to get CHANGE_ID set or means to post/update comments to PR other than success or Fail</p>