readme: document v0.12.0 and the ceph versions it supports
readme: document v0.12.0 and the ceph versions it supports

Also, change the hint regarding build tags to use "pacific" as it
a fully supported ceph version.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph