add readme installation
add readme installation

RuanShan/spree_alipay,derekluo/spree_alipay,dgross881/spree_alipay,derekluo/spree_alipay,dgross881/spree_alipay,RuanShan/spree_alipay,derekluo/spree_alipay,dgross881/spree_alipay,RuanShan/spree_alipay