docs(README): 补充链表专题题目

JalanJiang · JalanJiang · commit 26b87f14e847 · 2020-01-15T23:40:05.000+08:00
diff --git a/README.md b/README.md
@@ -33,4 +33,5 @@ LeetCode 刷题小组，组织代号 *Punch*，缘起我的 LeetCode 解题本 [
 
 ### 第一期：链表
 
-- [链表的中间结点](https://leetcode-cn.com/problems/middle-of-the-linked-list/)
+- [链表的中间结点](https://leetcode-cn.com/problems/middle-of-the-linked-list/)
+- [环形链表](https://leetcode-cn.com/problems/linked-list-cycle/)
