Add stale bot (#221)

Signed-off-by: Lukas Kämmerling <[email protected]>

.github/workflows/bot_stale.yml

@@ -0,0 +1,19 @@
+name: "Close stale issues"
+on:
+  schedule:
+    - cron: "30 12 * * *"
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          stale-issue-message: 'This issue has been marked as stale because it has not had recent activity. The bot will close the issue if no further action occurs.'
+          exempt-issue-label: 'pinned'
+          stale-issue-label: 'stale'
+          stale-pr-message: 'This PR has been marked as stale because it has not had recent activity. The bot will close the PR if no further action occurs.'
+          exempt-pr-label: 'pinned'
+          stale-pr-label: 'stale'
+          days-before-stale: 30
+          days-before-close: 5