update readme, add support for jdk 6 #1

Author: oldratlee

README.md

@@ -4,7 +4,7 @@ Java Dns Cache Manipulator(DCM)
 [![Build Status](https://travis-ci.org/alibaba/java-dns-cache-manipulator.svg?branch=master)](https://travis-ci.org/alibaba/java-dns-cache-manipulator) [![Coverage Status](https://coveralls.io/repos/alibaba/java-dns-cache-manipulator/badge.svg?branch=master)](https://coveralls.io/r/alibaba/java-dns-cache-manipulator?branch=master) 
 [![GitHub issues](https://img.shields.io/github/issues/alibaba/java-dns-cache-manipulator.svg)](https://github.com/alibaba/java-dns-cache-manipulator/issues) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.alibaba/dns-cache-manipulator/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.alibaba/dns-cache-manipulator/) [![License](https://img.shields.io/badge/license-Apache%202-blue.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
 
-:point_right: 通过代码直接设置`Java`的`DNS`（实际上设置的是`DNS Cache`），支持`JDK 7+`。
+:point_right: 通过代码直接设置`Java`的`DNS`（实际上设置的是`DNS Cache`），支持`JDK 6+`。
 
 :wrench: 功能
 ----------------------------
@@ -110,7 +110,7 @@ public void beforeClass() throws Exception {
 
 通过查看类`InetAddress`的实现可以确定：通过以`addressCache`字段为锁的`synchronized`块来保证线程安全。
 
-关键代码如下：
+其中关键代码（`JDK 7`）如下：
 
 ```java
 /*
@@ -138,3 +138,7 @@ private static void cacheAddresses(String hostname,
 - [tanhaichao](https://github.com/tanhaichao)的[`javahost`项目](https://github.com/tanhaichao/javahost)，
     该项目的[使用文档](http://leopard.io/modules/javahost)。  
     本项目如何设置`Java DNS Cache`的解法来自该项目。刚开始在持续集成项目中碰到`host`绑定的问题时，也是使用该项目来解决的 :+1:
+- 类`InetAddress`的源代码：
+    - `JDK 6`的[`InetAddress`](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b27/java/net/InetAddress.java#InetAddress.CacheEntry)
+    - `JDK 7`的[`InetAddress`](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/net/InetAddress.java#InetAddress.CacheEntry)
+    - `JDK 8`的[`InetAddress`](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8-b132/java/net/InetAddress.java#InetAddress.CacheEntry)