Documentation for runtime dynamic forms

[dflock]

I'd like to see documentation for dynamic forms - i.e. adding fields to a form at runtime.

For example: you have a form that collects URLs. This form allows the user to click a "Add URL" button to add more URL fields, to collect an additional URL. The user can click this button as many times as they like to add more URLs. These are collected in a URLs array in the model.