Prevent example from failing on unmerged PRs

This should prevent users from experiencing sorenlouv#127 when simply following the README example.

README.md

@@ -14,6 +14,7 @@ on:
 jobs:
   backport:
     name: Backport PR
+    if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
       - name: Backport Action