Document your code
Every project on GitHub comes with a version-controlled wiki to give your documentation the high level of care it deserves. It’s easy to create well-maintained, Markdown or rich text documentation alongside your code.
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This is the Cataclysm: DDA wiki for developers.
If you want to help contribute, a good place to start is by reading the Guide to adding new content to CDDA for first time contributors which covers setting yourself up to contribute and common JSON editing tasks.
If you're a programmer, you should read the guide above and the contributor guidelines.