Add .github/CONTRIBUTING.md

[musoke]

a file named .github/CONTRIBUTING.md in this repository will be used to give contributors instructions when creating PRs and issues in any repo in the OpenBeta organisation.

We can use that as an addition link to https://docs.openbeta.io/how-to-contribute/overview

https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file#supported-file-types
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors