doc: add happy number

Author: MoigeMatino

README.md

@@ -39,6 +39,7 @@ If you find this repo helpful, please consider giving it a star 🌟 to show you
 [BONUS: find duplicate number](https://github.com/MoigeMatino/data-structures-algorithms-python/tree/main/arrays_and_strings/find_duplicate_number)  
 [BONUS: first bad version](https://github.com/MoigeMatino/data-structures-algorithms-python/tree/main/arrays_and_strings/first_bad_version)  
 [BONUS: container with most water](https://github.com/MoigeMatino/data-structures-algorithms-python/tree/main/arrays_and_strings/container_with_most_water)  
+[BONUS: happy number](https://github.com/MoigeMatino/data-structures-algorithms-python/tree/main/arrays_and_strings/happy_number) * two pointers approach
 
 ## 2. Linked Lists