During the dev Summit 2019/2, there was a consensus to mark stale PR
after 60 days.
This change is adding the stale bot configuration required for this.
The stale bot has already has access to the Prometheus organization. It
does _not_ comment and does _not_ close the stale pull request. It just
adds a label 'stale'.
This is already done in the collectd_exporter repository and there it
works as expected.
https://docs.google.com/document/d/1VVxx9DzpJPDgOZpZ5TtSHBRPuG5Fr3Vr6EFh8XuUpgs/edit
Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>
