feat(issues): add stale bot (#3366)

Author: engai

.github/stale.yml

@@ -0,0 +1,8 @@
+daysUntilStale: 60
+daysUntilClose: 7
+staleLabel: stale
+markComment: >
+  This issue has been automatically marked as stale because it has not had recent activity.
+  It will be closed in 7 days if no further activity occurs. If you still need help with your issue,
+  please comment and we will reply to you shortly. Thank you for your contributions to the Salesforce Lightning Design System.
+closeComment: false

.gitignore

@@ -16,6 +16,7 @@
 !/.sldsrc.defaults.yml
 !/.stylestatsrc
 !/.travis.yml
+!/.github
 
 # Add a .gitkeep file in an empty
 # directory to check it in the repository