Added rotate string question

Author: Anuvab-Chakraborty

String/README.md

@@ -100,3 +100,5 @@ console.log(firstName.concat(lastName, " is a", " Best"));
 - [String Compression](https://leetcode.com/problems/string-compression)
 - [Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string)
 - [Reverse Vowels of a String](https://leetcode.com/problems/reverse-vowels-of-a-string)
+- [Rotate String](https://leetcode.com/problems/rotate-string)
+