Adding rabin-karp algo for string matching

[prasannakroy]

Have you read the Contributing Guidelines on Pull Requests?

Yes, I have read and followed the mentioned guidelines.

Description

I saw that there was only KMP algorithm mentioned here in string algos column and hence decided to add Rabin-Karp as well because although KMP is a self sufficient algo but having knowledge of Rabin-Karp and rolling hash is always an advantage.

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

No existing issue.

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