chore: add Update changelog action

.github/workflows/changelog.yml

@@ -0,0 +1,30 @@
+name: add PR to changelog
+
+on:
+  # needs read/write GH token, do *not* execute arbitrary code from PR
+  pull_request_target:
+    types: [closed]
+
+jobs:
+  update-changelog:
+    if: |
+      github.event.pull_request.merged == true &&
+      contains(github.event.pull_request.labels.*.name, 'changelog') &&
+      github.base_ref == 'master'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Update changelog
+        run: |
+          set -euxo pipefail
+          escaped_link=$(sed -e 's/[\/&]/\\&/g' <<'EOF'
+          [${{ github.event.pull_request.title}}](${{ github.event.pull_request.html_url }})
+          EOF
+          )
+          # insert link below first dashes line (https://stackoverflow.com/a/9453461/161659)
+          sed -i "0,/^---*/s/^---*/\0\n\n* $escaped_link./" RELEASES.md
+          # commit as github-actions bot (https://github.com/orgs/community/discussions/26560#discussioncomment-3252339)
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git config user.name "github-actions[bot]"
+          git commit -i RELEASES.md -m "update changelog"
+          git push