add changelog action (#298)

* add changelog action

.github/workflows/changelog.yml

@@ -0,0 +1,19 @@
+name: Changelog
+
+on:
+  pull_request:
+    types: [labeled, unlabeled, opened, synchronize, reopened]
+
+jobs:
+  changelog:
+    name: Confirm changelog entry
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+          submodules: true
+      - name: Grep for PR number in CHANGELOG.md
+        run: grep -P '\[[^\]]*#${{github.event.number}}[,\]]' CHANGELOG.md
+        if: ${{ !contains(github.event.pull_request.labels.*.name, 'no-changelog-entry-needed') }}