README: move some stuff around
README: move some stuff around

Add roadmap to the bottom and simplify the alpha warning.

coreos/etcd-operator,coreos/etcd-operator,fatduo/etcd-operator,icyxieex/operator,hongchaodeng/etcd-operator,icyxieex/operator,fatduo/etcd-operator,hongchaodeng/etcd-operator