ci: label PRs when merged

.github/labeler_merged.yml

@@ -0,0 +1,4 @@
+needs changelog:
+  - all: ['!CHANGELOG.md']
+needs README:
+  - all: ['!README.md']

.github/workflows/pr_merged.yml

@@ -0,0 +1,15 @@
+name: PR merged
+on:
+  pull_request_target:
+    types: [closed]
+
+jobs:
+  label-merged:
+    name: Changelog needed
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.merged == true
+    steps:
+    - uses: actions/labeler@main
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        configuration-path: .github/labeler_merged.yml