Merge pull request jenkinsci#85 from janinko/master

Update README

README.md

@@ -68,6 +68,7 @@ For more details, see https://wiki.jenkins-ci.org/display/JENKINS/GitHub+pull+re
   * If you **just** want to build PRs, set ``refspec`` to ``+refs/pull/*:refs/remotes/origin/pr/*``
   * If you want to build PRs **and** branches, set ``refspec`` to ``+refs/heads/*:refs/remotes/origin/* +refs/pull/*:refs/remotes/origin/pr/*`` (see note below about [parameterized builds](#parameterized-builds))
 * In ``Branch Specifier``, enter ``${sha1}`` instead of the default ``*/master``.
+* If you want to use the actual commit in the pull request, use ``${ghprbActualCommit}`` instead of ``${sha1}``
 * Under ``Build Triggers``, check ``GitHub pull requests builder``.
   * Add admins for this specific job.  
   * If you want to use GitHub hooks for automatic testing, read the help for ``Use github hooks for build triggering`` in job configuration. Then you can check the checkbox.