Add .github/stale.yml

After resolving a issue, we usually keep it open for about a week to give users some more time for feedback and further questions. This is especially true for issues with the `type: Notice`, `type: Question`, `type: Discussion` and `type: Invalid` labels. After 7 days with no interaction, [Probot](https://probot.github.io/)'s [Stale](https://probot.github.io/apps/stale/) bot (@probot-stale) adds the `info: Stale` label to the issue to ask the participants whether the issue has been resolved. If no more activity occurs, the issue will be automatically closed by @probot-stale 2 days later.

.github/stale.yml

@@ -0,0 +1,17 @@
+daysUntilStale: 7
+daysUntilClose: 2
+exemptLabels:
+  - "type: Bug"
+  - "type: Enhancement"
+  - "type: Feature"
+  - "type: Idea"
+  - "type: Release"
+  - "info: Pinned"
+staleLabel: "info: Stale"
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed in two days if no further activity
+  occurs. Thank you for your contributions! :+1:
+unmarkComment: false
+closeComment: false
+only: issues