This is the basic Tutorial from guides.rubyonrails.org/getting_started.html
Things you may want to cover:
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Ruby version
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System dependencies
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Configuration
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Database creation
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Database initialization
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How to run the test suite
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Services (job queues, cache servers, search engines, etc.)
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Deployment instructions
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…
Please feel free to use a different markup language if you do not plan to run rake doc:app.