Add Probot Stale configuration file

This is a configuration of Blender mirror on Github which automatically
closes Pull requests and gives developers instructions how to submit patch
to an official code review.

Differential Revision: https://developer.blender.org/D6027

.github/stale.yml

@@ -0,0 +1,22 @@
+# Configuration for probot-stale - https://github.com/probot/stale
+# This file is used on Blender's GitHub mirror to automatically close any pull request
+# and invite contributors to join the official development platform on blender.org
+
+# Number of days of inactivity before an Issue or Pull Request becomes stale
+daysUntilStale: 1
+
+# Number of days of inactivity before an Issue or Pull Request with the stale label is closed.
+# Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+daysUntilClose: 1
+
+# Label to use when marking as stale
+staleLabel: stale
+
+# Comment to post when closing a stale Issue or Pull Request.
+closeComment: >
+  This issue has been automatically closed, because this repository is only
+  used as a mirror of git.blender.org. Blender development happens on
+  developer.blender.org.
+
+  To get started contributing code, please read:
+  https://wiki.blender.org/wiki/Process/Contributing_Code