docs: specify recommended Node.js version

[EasterRobert]

Updated README.md to include recommended Node.js version for better clarity during initial environment setup.

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Note

Low Risk
Documentation-only change; no runtime behavior or API surface is affected.

Overview
Adds a new Requirements section to README.md that explicitly calls out a recommended Node.js version (v16+) and the need for npm or yarn before the existing usage example.

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