fix url of document

Author: yosupo06

README.md

@@ -7,8 +7,14 @@ You may refer to the following links for more details:
 - [The Announcement on AtCoder](https://atcoder.jp/posts/518)
 - [The Announcement on Codeforces](https://codeforces.com/blog/entry/82400)
 - [Practice Contest](https://atcoder.jp/contests/practice2)
-- [AC Library Document(en)](https://atcoder.github.io/ac-library/document_en/)
-- [AC Library Document(ja)](https://atcoder.github.io/ac-library/document_ja/)
+- [AC Library Document(en)](https://atcoder.github.io/ac-library/production/document_en/)
+- [AC Library Document(ja)](https://atcoder.github.io/ac-library/production/document_ja/)
+
+The documents of master branch is as follows:
+
+- [AC Library Document(en, master)](https://atcoder.github.io/ac-library/master/document_en/)
+- [AC Library Document(ja, master)](https://atcoder.github.io/ac-library/master/document_ja/)
+
 
 ## Policy