Add workflow to verify CHANGELOG is always up-to-date

.github/workflows/verify.yml

@@ -0,0 +1,17 @@
+name: Verify
+on:
+  pull_request:
+    branches:
+      - master
+jobs:
+  changelog:
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+    - name: Check `CHANGELOG.md` has changes
+      run: |
+        ! git diff --exit-code master HEAD CHANGELOG.md
+    - name: Check `CHANGELOG.md` has PR author
+      if: ${{ github.event.pull_request.user.name != 'hecrj' }}
+      run: |
+        sed -n '/## \[Unreleased\]/,/^## /p' CHANGELOG.md | sed -n '/Many thanks to.../,//p' | grep '@${{ github.event.pull_request.user.name }}'