Add README with setup instructions

[morgangauth]

Summary

Replaces the placeholder README with a proper setup guide for this multi-language test repository.

What's included

• Prerequisites — lists required tools (Git, Python, Node.js, Rust) with links and minimum versions
• Setup steps — clone, Python, Node.js/TypeScript, Rust, and environment variable instructions
• Repository structure table — maps every notable file to a short description
• Batch changes section — explains how to run the Sourcegraph batch change specs (hello-world.batch.yaml, hello-world2.batch.yaml)
• Code ownership section — references CODEOWNERS and explains the ownership rules