[README] Add contribution guide reference to README

[IanHoang]

To streamline the contribution experience, we should add the official contribution guide to the README so that users can reference it when creating PRs: https://opensearch.org/docs/latest/benchmark/user-guide/contributing-workloads/

[safimuhammad]

hey, can I get this one done?

[IanHoang]

@safimuhammad Yes! Feel free to cut out a PR for this and tag any maintainer to review. For future reference, you are welcome to pick out any issue and cut a PR out for it. No need to ask for permission 👍🏻

[safimuhammad]

Thanks alot.