document contribution process in readme

[serapath]

@todo

• how we work with github issues and pull requests
• explain how to use the module
• code of conduct
• how outsourcing and getting paid for it works
• add notes about testing/CI, github page and npm module
• add project documentation, but ideally make the app self documenting

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• inspiration1: https://github.com/datprotocol/working-group
• inspiration2: https://github.com/datprotocol/DEPs/blob/master/proposals/0001-dep-process.md