docs: add link to contribution guide

[braaar]

Description

If you use your browser's search function to search for 'contrib' on this repository's home page, you won't find any information about how to contribute. I myself was unaware of the existing contribution guide in TypeStack/.github until recently

Proposed solution

We should add a link to CONTRIBUTING.md in this repository's readme, or perhaps make a dedicated CONTRIBUTING.md here.

[github-actions]

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