Description

Added fibonacci number generator

Checklist:

[x] My code follows the style guidelines of this project
[x] I have added my code in its respective language folder.
[x] I have mentioned a short description about the code at the beginning of the program.
[x] I have performed a self-review of my own code.
[x] I have commented my code, particularly in hard-to-understand areas.
[x] My changes generate no new warnings.
[x] I have not deleted any files that already exist in the repository.