docs: complete README with install and quick-start guide

[jamilagibson]

Context

The current README is a placeholder. For npm packaging, developers need clear install instructions targeted at adding Reactime Native to an existing RN project.

Required sections

• Installation (npm install reactime-native --save-dev)
• Setup — adding initReactimeNative() to the RN app entry point
• Starting the server (per the agreed distribution strategy)
• Opening the browser UI
• Configuration options
• Troubleshooting (common connection issues, IP resolution)
• Contributing guide link
• License

References

• Decision 7 in CLAUDE.md — IP resolution and EXPO_PUBLIC_WS_HOST env var (must be documented)