Add .github/stale.yml to auto close stale issues and PRs.

See https://probot.github.io/apps/stale/

Change-Id: I8176eeb21dbcd516f1d6f7b1e8c28602cdd9332a

.github/stale.yml

+# 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: wontfix
+# 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
+  activity for 6 months. It will be closed 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