Feature: gather domain knowledge in a separate file like DOMAIN.md

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If you work with really big or long term product it usually requires gathering of business or domain knowledge from the real world. And often domain quirks drives solution of even architecture. This information lies outside of the code in the real world. In the real products this business knowledge usually in the head of stakeholders and implicitly scattered across code. When you write the code you can clarify decisions and rationale with links to domain knowledge. This file should be always actual and populated from research phases.