Don't run changelog checker until PR is ready for review (#315)

Author: nmuesch

.github/workflows/changelog.yaml

@@ -7,13 +7,14 @@ on: # yamllint disable-line rule:truthy
       - opened
       - synchronize
       - reopened
+      - ready_for_review
 
 jobs:
   changelog:
     runs-on: ubuntu-latest
     steps:
       - name: Check changelog labels
-        if: false == contains(join(github.event.pull_request.labels.*.name, ','), 'changelog/')
+        if: github.event.pull_request.draft == false && false == contains(join(github.event.pull_request.labels.*.name, ','), 'changelog/')
         run: |-
           echo "::error Add 'changelog/*' label";
           exit 1;