Add auto update GitHub action

Enable https://github.com/tibdex/auto-update to simplify automatic
merging of PRs.

Signed-off-by: Till Maas <opensource@till.name>

.github/workflows/auto-update.yml

@@ -0,0 +1,23 @@
+---
+name: Auto-update
+# Auto-update only listens to `push` events.
+# If a pull request is already outdated when enabling auto-merge, manually
+# click on the "Update branch" button a first time to avoid having to wait for
+# another commit to land on the base branch for the pull request to be updated.
+# on: push
+# If pull requests are always based on the same branches, only triggering the
+# workflow when these branches receive new commits will minimize the workflow
+# execution.
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  Auto:
+    name: Auto-update
+    runs-on: ubuntu-latest
+    steps:
+      - uses: tibdex/auto-update@v2
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}