extend stale-bot's deadline (#1224)

Add a note in about using the 'pinned' label for prevent stalebot from
closing your issue.

.github/stale.yml

 # Number of days of inactivity before an issue becomes stale
-daysUntilStale: 60
+daysUntilStale: 100
 # Number of days of inactivity before a stale issue is closed
-daysUntilClose: 7
+daysUntilClose: 21
 # Issues with these labels will never be considered stale
 exemptLabels:
   - pinned

CONTRIBUTING.md

@@ -20,6 +20,9 @@ the faster we will be able to help you. Check out
 Please be kind. :smile: Remember that CoreDNS comes at no cost to you, and you're
 getting free help.
 
+We are using [Stable Bot](https://github.com/probot/stale) to close inactive issues. If you feel an
+issue should not be closed you can add the "pinned" label to it or just update the bug ever now or
+then.
 
 ## Minor improvements and new tests