adding sieve of eratosthenes in go

[lucblassel]

Have you read the Contribution Guidelines?

Yes

Description

Implementing the Sieve of Eratosthenes prime number finder in Go.

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 (Mention Issue no. like #xxx)

Fixes Issue #172