Added README and 3Sum files

[deek121477]

Pull Request Template

Question number -15

Issue no.(must) - #27

Self Check(Tick After Making pull Request)

• [x] This issue was assigned to me.
• [x] One Change in one Pull Request
• [x] My file is in proper folder (Name of folder should be Question number) (E.g. 120)
• [x] I am following clean code and Documentation and my code is well linted with flake8.
• [x] I have added README.md (With question description)
• [ ] I have added sufficient comments in my code

If issue was not assigned to you Please don't make a PR. It will mark as invalid.