Revise README

Author: xirc

lib/cpalgo/ds/README.md

@@ -8,8 +8,20 @@ a.k.a Union Find Tree
 - [道路の老朽化対策について - AtCoder ABC040D](https://atcoder.jp/contests/abc040/tasks/abc040_d)
 
 ## Binary Indexed Tree
-🚧  
-a.k.a Fenwick Tree
+
+Binary Indexed Tree (Fenwick Tree) is a data structure.
+- It can calculate a cumulative function in the given range `[l,r)` in `O(logN)`.
+- It can update an element of A in `O(logN)`.
+
+- [Binary Indexed Tree | C++ code](binary_indexed_tree.hpp)
+- [Binary Indexed Tree 2D | C++ code](binary_indexed_tree_2d.hpp)
+
+### References in English
+- [Binary Indexed Trees](https://www.topcoder.com/thrive/articles/Binary%20Indexed%20Trees)
+- [Fenwick Tree - Competitive Programming Algorithms](https://cp-algorithms.com/data_structures/fenwick.html)
+
+### References in Japanese
+- [Binary Indexed Tree のはなし](http://hos.ac/slides/20140319_bit.pdf)
 
 ## Minimum Queue
 🚧WIP