add stale bot configuration (#2790)

.github/stale.yml

+# Configuration for probot-stale - https://github.com/probot/stale
+
+# General configuration
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 60
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - bug
+  - discussion
+  - enhancement
+  - feature
+  - help wanted
+  - info
+  - need investigation
+  - tips
+
+# Set to true to ignore issues in a project (defaults to false)
+exemptProjects: true
+# Set to true to ignore issues in a milestone (defaults to false)
+exemptMilestones: true
+# Set to true to ignore issues with an assignee (defaults to false)
+exemptAssignees: true
+# Label to use when marking an issue as stale
+staleLabel: stale
+
+# Pull request specific configuration
+pulls:
+  # Number of days of inactivity before a stale Issue or Pull Request is closed.
+  # Set to false to disable. If disabled, issues still need to be closed manually, but will remain marked as stale.
+  daysUntilClose: 30
+  # Comment to post when marking as stale. Set to `false` to disable
+  markComment: >
+    This pull request has been automatically marked as stale because it has not had activity
+    in the last 90 days. It will be closed in 30 days if no further activity occurs. Please
+    feel free to give a status update now, ping for review, or re-open when it's ready.
+    Thank you for your contributions!
+  # Comment to post when closing a stale Issue or Pull Request.
+  closeComment: >
+     This pull request has been automatically closed because it has not had
+     activity in the last 30 days. Please feel free to give a status update now, ping for review, or re-open when it's ready.
+     Thank you for your contributions!
+  # Limit the number of actions per hour, from 1-30. Default is 30
+  limitPerRun: 1
+
+# Issue specific configuration
+issues:
+  # Number of days of inactivity before a stale Issue or Pull Request is closed.
+  daysUntilClose: 14
+  # Comment to post when marking as stale. Set to `false` to disable
+  markComment: >
+    This issue has been automatically marked as stale because it has not had activity in the
+    last 90 days. It will be closed in 14 days unless it is tagged "help wanted" or other activity
+    occurs. Thank you for your contributions.
+  # Comment to post when closing a stale Issue or Pull Request.
+  closeComment: >
+    This issue has been automatically closed because it has not had activity in the
+    last 14 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted".
+    Thank you for your contributions.
+  # Limit the number of actions per hour, from 1-30. Default is 30
+  limitPerRun: 1