doc: add Mergify badge to README
doc: add Mergify badge to README

We heavily use the service for Open Source communities from Mergify. It
is probably nice to promote them a little in our README.

Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi