Create stale.yml (#1038)

Summary:
Setting up a [stale bot](https://github.com/probot/stale) to help us stay on top of all our issues.
Pull Request resolved: https://github.com/facebook/flipper/pull/1038

Test Plan: testinprod

Reviewed By: jknoxville

Differential Revision: D21228297

Pulled By: mweststrate

fbshipit-source-id: 0668eb3de9e888799a7904305dccf1862bbca759

.github/stale.yml

@@ -0,0 +1,21 @@
+# Configuration for probot-stale - https://github.com/probot/stale
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 90
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 14
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - Good first issue
+  - "Type: Discussion"
+  - Backlog
+# 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: >
+  Hey there, it looks like there has been no activity on this issue recently. Has the issue been fixed, or does it still require the community's attention? This issue may be closed if no further activity occurs.
+  You may also label this issue as a "Discussion" or add it to the "Backlog" and I will leave it open.
+  Thank you for your contributions.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: >
+  Closing this issue after a prolonged period of inactivity. If this issue is still present in the latest release, please feel free to create a new issue with up-to-date information.
+only: issues