Add Stale bot config

Added .github/stale.yml which Stale bot,
https://github.com/apps/stale, uses as a config.

6 months of inactivity must pass to mark an issue as stale
and 1 month of inactivity before a stale issue is closed.

See https://lists.ozlabs.org/pipermail/openbmc/2019-February/015022.html

Tested: Changed to 1 day and 1 day and pushed this config
        to a private repo where I had installed Stale bot.
        See gtmills/test-stale-bot#1
Change-Id: Ibc124913ed471bff6d52ca1e6c39b16584310ee5
Signed-off-by: Gunnar Mills <[email protected]>

Author: gtmills

.github/stale.yml

@@ -0,0 +1,23 @@
+# Stale bot config https://github.com/apps/stale
+
+# Number of days of inactivity before an issue becomes stale (6 months)
+daysUntilStale: 183
+
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 30
+
+# Label to use when marking an issue as stale
+staleLabel: stale
+
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because no activity has
+  occurred in the last 6 months. It will be closed if no activity occurs in the
+  next 30 days. If this issue should not be closed please add a comment.
+  Thank you for your understanding and contributions.
+
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: >
+  This issue has been closed because no activity has occurred in the last 7
+  months. Please reopen if this issue should not have been closed. Thank you
+  for your contributions.