📥 Add installation instructions and distribution methods

[szmyty]

Title: 📥 Add installation instructions and distribution methods

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Description

Document how users can install and use Renderflow.

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Goals

• Provide simple install methods
• Improve onboarding experience

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Requirements

Include instructions for:

• Downloading from GitHub Releases
• Running binary directly
• Optional:

cargo install --path .

• Usage examples:

renderflow input.md

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Acceptance Criteria

• Instructions clear and concise
• Covers common platforms
• Easy to follow