Add project overview and usage in README

[leynos]

Summary

• document installation, usage and development commands
• detail project structure for new contributors

Testing

• markdownlint README.md
• ruff format --check
• ruff check
• pyright
• pytest -q

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https://chatgpt.com/codex/tasks/task_e_684635432bf08322b5f5e62be26494ca

Summary by Sourcery

Enhance README with a comprehensive project overview, usage guidance, installation steps, development workflow, and structure details

Documentation:

• Add Features and Requirements sections
• Add Installation and Usage instructions with examples
• Document Development workflow with formatting, linting, type checking, and testing commands
• Outline Project Structure and reference License

[sourcery-ai]

Reviewer's Guide

This PR refactors the README.md by restructuring and expanding the documentation: it adds clear sections for features, requirements, installation, usage examples (including concurrency syntax and error output), development workflows with standardized commands, and contributor orientation via project structure and license details.

File-Level Changes

Change	Details	Files
Reorganized and expanded documentation sections	Split introduction into Features, Requirements, Installation, Usage Replaced single-line description with detailed bullets Inserted code blocks for install and usage examples	README.md
Enhanced CLI usage guidance	Added concurrency flag explanation and default behavior Included sample error output for invalid diagrams	README.md
Standardized development workflow	Specified linting, formatting, type-checking, and testing commands Described mock integration testing to avoid Node.js dependency	README.md
Added contributor orientation sections	Detailed project directory structure Referenced LICENSE file for licensing information	README.md

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[sourcery-ai]

Hey @leynos - I've reviewed your changes - here's some feedback:

• In the Installation section, briefly introduce what uv is and link to its docs at first mention to give new contributors context.
• In the Usage section, specify the default value for --concurrency so users know what happens when they omit that flag.
• Consider adding a full example invocation (with sample output) to the Usage section to illustrate a typical nixie run.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

---

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[leynos]

@sourcery-ai review

[sourcery-ai]

Hey @leynos - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

---

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• X
• Mastodon
• LinkedIn
• Facebook

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

[sourcery-ai]

suggestion: Consider adding installation instructions for end-users.

If nixie is meant for non-developers, please include standard installation steps (e.g., pip install nixie) in addition to development setup.