added probot configuration (#719)

this commit includes the configuration for the github-app
probot. more information can be found here:
https://probot.github.io/apps/stale/

.github/stale.yml

@@ -0,0 +1,25 @@
+# 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
+  - 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
+  recent activity. 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: >
+  This issue has been automatically closed because it has not had
+  recent activity. This probably means that it is not reproducible
+  or it has been fixed in a newer version. If it’s an enhancement
+  and hasn’t been taken on for so long, then it seems no one has
+  the time to implement this.
+  Please reopen if you still encounter this issue with the [latest stable version](https://www.veracrypt.fr/en/Downloads.html).
+  You can also contribute directly by providing a pull request.
+  Thank you!