chore: enable verification of changelog entries for PRs (#668)

Author: ianbotsf

.github/workflows/changelog-verification.yml

@@ -0,0 +1,24 @@
+name: Changelog verification
+
+on:
+  pull_request:
+    types: [ opened, synchronize, reopened, labeled, unlabeled ]
+    branches: [ main ]
+
+jobs:
+  changelog-verification:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check for changelog entry
+        if: ${{ !contains(github.event.pull_request.labels.*.name, 'no-changelog') }}
+        run: |
+          git fetch origin ${{ github.base_ref }} --depth 1 && \
+          git diff remotes/origin/${{ github.base_ref }} --name-only | grep -P "\.changes/[0-9a-f-]+\.json"
+      - name: Error message
+        if: ${{ failure() }}
+        run: |
+          echo "::error ::No new/updated changelog entry found in /.changes directory. Please either:"
+          echo "::error ::* Add a changelog entry (in most cases) –or–"
+          echo "::error ::* Add the 'no-changelog' label to this PR (in rare cases not warranting a changelog entry)"
+          exit 1