more install istio info (#311)

docs/case-studies/canary-blue-green.md

@@ -14,11 +14,13 @@ We will use the well-known [bookinfo](https://istio.io/latest/docs/examples/book
 
 ## Preparation
 
-Install the Istio cluster plugin.
+If your cluster haven't installed Istio. Install the Istio cluster plugin.
 ```shell
 vela addon enable istio
 ```
 
+Otherwise, you just need apply these 4 YAML files under this [path](https://github.com/oam-dev/kubevela/tree/master/vela-templates/addons/istio/definitions) 
+
 The default namespace needs to be labeled so that Istio will auto-inject sidecar.
 
 ```shell

i18n/zh/docusaurus-plugin-content-docs/version-v1.1/case-studies/canary-blue-green.md

@@ -16,11 +16,13 @@ KubeVela 后的应用交付模型（OAM）是一个从设计与实现上都高
 
 ## 准备工作
 
-开启 Istio 集群插件
+如果你的集群尚未安装 Istio，你可以通过以下命令，开启 Istio 集群插件
 ```shell
 vela addon enable istio
 ```
 
+如果你的集群已经已经安装 Istio，你只需 apply [该目录](https://github.com/oam-dev/kubevela/tree/master/vela-templates/addons/istio/definitions) 下的四个 YAML 文件来达到和上面开启集群插件一样的效果
+
 因为后面的例子运行在 default namespace，需要为 default namespace 打上 Istio 自动注入 sidecar 的标签。
 
 ```shell