README: add backporting instructions
README: add backporting instructions

Add instructions for how we maintain and release the stable release
series.

We'd been following this for a while, so let's get it written down.

fgal/ceph-ansible,ceph/ceph-ansible,bengland2/ceph-ansible,fgal/ceph-ansible,travmi/ceph-ansible,jtaleric/ceph-ansible,jtaleric/ceph-ansible,font/ceph-ansible,travmi/ceph-ansible,font/ceph-ansible,bengland2/ceph-ansible,ceph/ceph-ansible