Update README.md

Author: Snailclimb

README.md

@@ -62,12 +62,14 @@
 
 ### Java 集合框架
 
-* [这几道Java集合框架面试题几乎必问](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/这几道Java集合框架面试题几乎必问.md)
-* [Java 集合框架常见面试题总结](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/Java集合框架常见面试题总结.md)
-* [ArrayList 源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/ArrayList.md) 
-* [【面试必备】透过源码角度一步一步带你分析 ArrayList 扩容机制](https://github.com/Snailclimb/JavaGuide/blob/master/Java相关/ArrayList-Grow.md)    
-* [LinkedList 源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/LinkedList.md)   
-* [HashMap(JDK1.8)源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/HashMap.md)  
+* **常见问题总结：**
+  * [这几道Java集合框架面试题几乎必问](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/这几道Java集合框架面试题几乎必问.md)
+  * [Java 集合框架常见面试题总结](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/Java集合框架常见面试题总结.md)
+* **源码分析：**
+  * [ArrayList 源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/ArrayList.md) 
+  * [【面试必备】透过源码角度一步一步带你分析 ArrayList 扩容机制](https://github.com/Snailclimb/JavaGuide/blob/master/Java相关/ArrayList-Grow.md)    
+  * [LinkedList 源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/LinkedList.md)   
+  * [HashMap(JDK1.8)源码学习](https://github.com/Snailclimb/Java-Guide/blob/master/Java相关/HashMap.md)  
 
 ### Java 多线程