add stale-bot configuration (#82)

Add [Stale bot](https://github.com/probot/stale).

.github/stale.yml

@@ -0,0 +1,11 @@
+daysUntilStale: 60
+daysUntilClose: 7
+exemptLabels:
+  - pinned
+  - security
+staleLabel: stale
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed if no further activity occurs. Thank you
+  for your contributions.
+closeComment: false