doc: update README to make Node.js description clearer

Fixes: https://github.com/nodejs/node/issues/24872

This PR updates the README's original description:

Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine.

To:

Node.js is an open-source, cross-platform, JavaScript runtime environment. It executes JavaScript code outside of a browser

Based on the two sentence suggestion from the issue discussion.

Checklist

• documentation is changed or added
• commit message follows commit guidelines