Update README.md

leetcode104：二叉树的最大深度

Author: sisterAn

Diff for: README.md

@@ -76,6 +76,7 @@
 - [leetcode94：二叉树的中序遍历](https://github.com/sisterAn/JavaScript-Algorithms/issues/38)
 - [leetcode145：二叉树的后序遍历](https://github.com/sisterAn/JavaScript-Algorithms/issues/40)
 - [leetcode105：从前序与中序遍历序列构造二叉树](https://github.com/sisterAn/JavaScript-Algorithms/issues/41)
+- [leetcode104：二叉树的最大深度](https://github.com/sisterAn/JavaScript-Algorithms/issues/42)
 
 #### 编程题
 - [携程&蘑菇街&bilibili：手写数组去重、扁平化函数](https://github.com/sisterAn/JavaScript-Algorithms/issues/30)