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 <ndevos@redhat.com>

README.md

@@ -1,6 +1,7 @@
 # Ceph CSI
 
 [![GitHub release](https://img.shields.io/github/release/ceph/ceph-csi/all.svg)](https://github.com/ceph/ceph-csi/releases)
+[![Mergify Status](https://img.shields.io/endpoint.svg?url=https://api.mergify.com/v1/badges/ceph/ceph-csi&style=flat)](https://mergify.com)
 [![Go Report
 Card](https://goreportcard.com/badge/github.com/ceph/ceph-csi)](https://goreportcard.com/report/github.com/ceph/ceph-csi)
 [![TODOs](https://badgen.net/https/api.tickgit.com/badgen/github.com/ceph/ceph-csi/devel)](https://www.tickgit.com/browse?repo=github.com/ceph/ceph-csi&branch=devel)