Add github action requiring a diff to changelog.md.

.github/workflows/changelog.yml

@@ -0,0 +1,39 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - develop
+
+jobs:
+  # ------------------------------------------------------------------------------------------------------------------------------------------------
+  # Event `pull_request`: Compare the last commit of the main branch or last remote commit of the PR branch -> to the current commit of a PR branch.
+  # ------------------------------------------------------------------------------------------------------------------------------------------------
+  changed_files:
+    runs-on: ubuntu-latest 
+    name: Check CHANGELOG.md
+    steps:
+      - uses: actions/checkout@v4
+
+      # -----------------------------------------------------------------------------------------------------------
+      # Example 1
+      # -----------------------------------------------------------------------------------------------------------
+      - name: changelog modified
+        id: changelog-modified
+        uses: tj-actions/changed-files@v42
+        with:
+          files: CHANGELOG.md
+
+      - name: List all changed files
+        env:
+          ALL_CHANGED_FILES: ${{ steps.changelog-modified.outputs.all_changed_files }}
+        run: |
+          if [[ ${ALL_CHANGED_FILES[*]} =~ (^|[[:space:]])"CHANGELOG.md"($|[[:space:]]) ]];
+          then
+            echo "CHANGELOG.md was modified.";
+            exit 0;
+          else
+            echo "CHANGELOG.md was not modified.";
+            echo $ALL_CHANGED_FILES;
+            exit 1;
+          fi