create configuration file for probot-stale (#1360)

as discussed in #1350.

.github/stale.yml

@@ -0,0 +1,29 @@
+# Configuration for probot-stale - https://github.com/probot/stale
+
+# Number of days of inactivity before an issue or PR is considered stale
+daysUntilStale: 15
+
+# Number of days of inactivity before a stale issue or PR is closed
+daysUntilClose: 30
+
+# Issues or PRs with these labels will never be considered stale
+exemptLabels:
+  - up for grabs
+
+# Label to use when marking an issue or PR as stale
+staleLabel: waiting
+
+# Comment to post when marking an issue or PR as stale
+markComment: >
+  Hi all! This thread has not had any recent activity.
+  Are there any updates? Thanks!
+
+# Comment to post when closing a stale issue or PR
+closeComment: >
+  Hi everyone.
+  This thread is being closed as there was no response to the previous prompt.
+  However, please leave a comment whenever you're ready to resume,
+  so the thread can be reopened. Thanks again!
+
+# Limit to only `issues` or `pulls`
+only: pulls