Add a GH action that checks changelog (#3588)

.github/workflows/checks.yml

@@ -13,6 +13,19 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  changelog:
+    if: github.event_name == 'pull_request' && github.repository != 'OpenZeppelin/openzeppelin-contracts-upgradeable'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Check diff
+        run: |
+          git fetch origin ${{ github.base_ref }} --depth=1
+          if git diff --exit-code origin/${{ github.base_ref }} -- CHANGELOG.md ; then
+            echo 'Missing changelog entry'
+            exit 1
+          fi
+
   lint:
     if: github.repository != 'OpenZeppelin/openzeppelin-contracts-upgradeable'
     runs-on: ubuntu-latest