create stale PR workflow

Signed-off-by: Avelino <t@avelino.xxx>

.github/workflows/stale.yml

@@ -0,0 +1,22 @@
+name: PR auto close, if you stay more than 8 days open
+
+on:
+  workflow_dispatch:
+  schedule:
+  - cron: '*/15 * * * *'
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/stale@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-pr-message: >
+          This PR has been automatically marked as closed because it has not had
+          recent activity. They will wait 15 days for your interaction, after
+          that the PR will be closed.
+          Please read more in https://github.com/avelino/awesome-go/blob/master/CONTRIBUTING.md
+        stale-pr-label: 'stale'
+        days-before-stale: 15
+        days-before-close: 7