Add contributing guide (CONTRIBUTING.md)

[thegreatalxx]

Summary

Conductor has no CONTRIBUTING.md file, which makes it hard for first-time contributors to know how to get started — how to set up the dev environment, how to submit a PR, code style, etc.

What to write

A CONTRIBUTING.md file that covers:

1. Getting started — how to clone and run the project locally
2. 2. Project structure — quick overview of where plugins, tools, and config live
3. 3. How to add a plugin — the simplest contribution path
4. 4. PR process — branch naming, commit style, what to expect in review
5. 5. Code style — any linting/formatting rules

Why it's a good first issue

• Pure documentation, no code changes needed
• • Forces you to understand the project structure (great onboarding experience)
• • • High impact: every future contributor benefits from this

Notes

Look at how other TypeScript open source projects structure their CONTRIBUTING.md for reference. Keep it short and friendly — this project is built by a 13-year-old, the tone should match.