[mixxer]

Summary

Add a contributor guide for source builds, validation, and pull request preparation, together with a documentation index and README link. The guide does not assume the proposed Android tools or fork CI workflow have landed.

Validation

• Rebased on upstream 6560052a6f856855d3f71eee838fd66ccfa4285d; tested head 07a229bc49b58b13a989a0a35a041ee40d3ce6ad.
• Whitespace validation and relative documentation links checked.
• Linux CI: all 11 jobs passed, including this exact PR head on Node 24. Native kernel checks also passed for fix: preserve Java and Kotlin anonymous inheritance #1927 and fix(java): resolve calls through static fields #1949; integrated main passed on Node 22 and 24.

[colbymchenry]

Thanks for this, @mixxer! We've landed a contributing guide and docs index in #3081, built on the structure you laid out here and credited to you, so I'm closing this in its favor. It also adds where viewer changelog entries go and how maintainers land first contributions.

[mixxer]

Thanks @colbymchenry for adapting the guide to the current project conventions and landing it in #3081. I appreciate the credit!