Add stale workflow (#384)

Author: Danny McCormick

.github/workflows/stale.yml

@@ -0,0 +1,20 @@
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+  - cron: "0 * * * *"
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/stale@v1
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        stale-issue-message: 'This issue has had no activity in 90 days. Please comment if it is not actually stale'
+        stale-issue-label: 'stale'
+        days-before-stale: 90
+        days-before-close: 7
+        exempt-pr-label: 'no-stale'