added cpp code for shell sort

[anshmishra010]

Have you read the Contributing Guidelines on Pull Requests?

yes

Description

added cpp code for shell sort

Checklist

• [x] I've read the contribution guidelines.
• [x] I've checked the issue list before deciding what to submit.
• [x] I've edited the README.md and link to my code.

Related Issues or Pull Requests

304

[anshmishra010]

@siddharth25pandey please see this

[anshmishra010]

@siddharth25pandey done