title: Jenkins Pipeline - automatic build triggering on GitHub PRs?
body: <p>I am converting existing Jenkins jobs in the new Pipeline implementation using Jenkinsfiles. However, what is not obvious at all is how to do the most basic triggering on pull requests. My existing BuildFlows are configured to build GitHub PR branches automatically when ever a PR is opened against the master branch. How do you set this up via Pipeline as Code?</p>
  
  <p>Since the Jenkinsfile lives in SCM, the pull request triggering obviously cannot be defined in that file (chicken or the egg problem...).</p>
  
  <p>I've tried using the GitHub Organization folder plugin, but it's not clearly documented and still isn't obvious how to enable automatic building of PRs against master. The plugin scans my repo and creates jobs for <strong>existing</strong> branches, but I don't see how I can get it to build PRs <strong>as they are opened</strong>.</p>
  
  <p>Anyone figured out how to achieve this? I really like the new Pipeline as Code idea, but the configuration is very non-intuitive.</p>