Update apollo client readme

apollo-client/README.md

-## Maven Dependency
+## I. Prerequisite
+
+### I.I Mandatory Setup
+Apollo client requires `AppId` and `Environment` information available to function properly, so please read the following and configure them properly:
+
+#### 1. AppId
+
+AppId is the identity for the application, which is a key information to retrieve the config from server.
+
+AppId information should be put in `classpath:/META-INF/app.properties` with its key as `app.id`.
+
+For example, you could place the file as the following screenshot:
+
+![app.properties example](doc/pic/app-id-location.png)
+
+And config the file as:
+
+> app.id=YOUR-APP-ID
+
+#### 2. Environment
+
+Apollo supports config by multiple environments, so environment is another key information to retrieve the config from server.
+
+Environment could be configured in 3 ways:
+
+1. As Java System Property
+	* You could specify environment as java system property `env`
+	* For example, when starting the java application, it can be configured via `-Denv=YOUR-ENVIRONMENT`
+	* Please note the key should be lower case
+
+2. As OS System Environment
+	* You could also specify environment as system environment `ENV`
+	* Please note the key should be UPPER CASE
+
+3. As Property File
+	* You could create a file `/opt/settings/server.properties` on the target machine
+	* And specify the environment in the file as `env=YOUR-ENVIRONMENT`
+	* Please note the key should be lower case
+
+### I.II Optional Setup
+
+#### Cluster
+
+Apollo supports config separated by clusters, which means for one appId and one environment, you could have different configs.
+
+If you need this functionality, you could specify the cluster as follows:
+
+1. As Java System Property
+	* You could specify environment as java system property `apollo.cluster`
+	* For example, when starting the java application, it can be configured via `-Dapollo.cluster=xxx`
+	* Please note the key should be lower case
+2. As Property file
+	* You could create a file `/opt/settings/server.properties` on the target machine
+	* And specify the environment in the file as `idc=xxx`
+	* Please note the key should be lower case
+
+##### Cluster Precedence
+
+1. If both `apollo.cluster` and `idc` are specified:
+	* We will first try to load config from cluster specified as `apollo.cluster`
+	* If not found, we will fall back to cluster specified as `idc`
+	* If still not found, we will fall back to the default cluster `default`
+
+2. If only `apollo.cluster` is specified:
+	* We will first try to load config from cluster specified as `apollo.cluster`
+	* If not found, we will fall back to the default cluster `default`
+
+3. If only `idc` is specified:
+	* We will first try to load config from cluster specified as `idc`
+	* If not found, we will fall back to the default cluster `default`
+
+4. If neither `apollo.cluster` nor `idc` is specified:
+	* We will load config from the default cluster `default`
+
+## II. Maven Dependency
 		<dependency>
 			<groupId>com.ctrip.framework.apollo</groupId>
 			<artifactId>apollo-client</artifactId>
 			<version>0.0.1</version>
 		</dependency>
 
-##	Client Usage
+## III. Client Usage
 
 ### 1. Load config from default namespace(application)
 ```java