init client readme

apollo-client/README.md

+## Maven Dependency
+		<dependency>
+			<groupId>com.ctrip.apollo</groupId>
+			<artifactId>apollo-client</artifactId>
+			<version>0.0.1-SNAPSHOT</version>
+		</dependency>
+
+##	Client Usage
+
+### 1. Load config from default namespace(application)
+```java
+Config config = ConfigService.getAppConfig();
+String someKey = "someKeyFromDefaultNamespace";
+String someDefaultValue = "someDefaultValueForTheKey";
+System.out.println(config.getProperty(someKey, someDefaultValue));
+```
+
+### 2. Register config change listener
+```java
+Config config = ConfigService.getAppConfig();
+config.addChangeListener(new ConfigChangeListener() {
+	@Override
+	public void onChange(ConfigChangeEvent changeEvent) {
+		System.out.println("Changes for namespace " + changeEvent.getNamespace());
+		for (String key : changeEvent.changedKeys()) {
+			ConfigChange change = changeEvent.getChange(key);
+			System.out.println(String.format("Change - key: %s, oldValue: %s, newValue: %s, changeType: %s", change.getPropertyName(), change.getOldValue(), change.getNewValue(), change.getChangeType()));
+		}
+	}
+});
+```
+
+### 3. Load config from public namespace
+```java
+String somePublicNamespace = "CAT";
+Config config = ConfigService.getConfig(somePublicNamespace);
+String someKey = "someKeyFromPublicNamespace";
+String someDefaultValue = "someDefaultValueForTheKey";
+System.out.println(config.getProperty(someKey, someDefaultValue));
+```
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