Add Stale bot integration

This change enables the Probot stale issues bot to automatically close
inactive issues.

[skip ci]

.github/stale.yml

@@ -0,0 +1,17 @@
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 180
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 14
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - pinned
+  - security
+# Label to use when marking an issue as stale
+staleLabel: stale
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity in the past 6 months. It will be closed within 2 weeks
+  if no further activity occurs. Thank you for your contributions.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false