add stale bot
add stale bot

By popular demand, add the stale bot which we'are already using at
Podman.  The bot will remind us every 30 days of inactivity on specific
issues or pull requests and has turned into a useful tool to keep things
on the radar.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

TomSweeneyRedHat/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah,rhatdan/buildah,projectatomic/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,nalind/buildah-1,nalind/buildah,projectatomic/buildah