docs: document secrets, custom_runtime_options, and remaining source/sink arguments in README - #204

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