update local building instructions in README.md - #284

Netlify credentials not provided, not deployable

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/save@v4, actions/cache@v4, actions/checkout@v2, actions/setup-python@v3, nwtgck/actions-netlify@v2.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/