Create sieve.cpp

[Nandini2901]

Description

Please include a summary of the program i.e what problem it solves.
The program prints out the all the prime numbers upto a given number. The time complexity of the program is O(n log log n) which is much faster as compared to the iterative approach which takes O(n) time.

Checklist:

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