Add Node Version requirements in README.md

[Moe-Salim91156]

The current README.md for the MDN Content repository does not mention the required Node.js version node >= 24. Users running Node.js <24 encounter the following error when running npm install:
npm error code EBADENGINE
npm error engine Unsupported engine
npm error engine Not compatible with your version of node/npm: @mdn/content@1.0.0
npm error notsup Not compatible with your version of node/npm: @mdn/content@1.0.0
npm error notsup Required: {"node":">=24"}
npm error notsup Actual: {"npm":"10.9.2","node":"v22.14.0"}