Add probot Stale integration (DataDog#161)

* Add probot Stale integration

* Updateprobot Stale integration

Author: agocs

.github/stale.yaml

@@ -0,0 +1,18 @@
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 60
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 7
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - pinned
+# 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 and it will be closed
+  if no further activity occurs. Thank you for your contributions! You can 
+  also find us in the \#serverless channel from the 
+  [Datadog community Slack](https://chat.datadoghq.com/).
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false
+