feat(netlify-cms-widget-list): add variable type definitions to list widget (#1857)

website/content/docs/beta-features.md

@@ -8,6 +8,84 @@ We run new functionality in an open beta format from time to time. That means th
 
 **Use these features at your own risk.**
 
+## List Widget: Variable Types
+Before this feature, the [list widget](/docs/widgets/#list) allowed a set of fields to be repeated, but every list item had the same set of fields available. With variable types, multiple named sets of fields can be defined, which opens the door to highly flexible content authoring (even page building) in Netlify CMS.
+
+**Note: this feature does not yet support previews, and will not output anything in the preview
+pane.**
+
+To use variable types in the list widget, update your field configuration as follows:
+
+1. Instead of defining your list fields under `fields` or `field`, define them under `types`.  Similar to `fields`, `types` must be an array of field definition objects.
+2. Each field definition under `types` must use the `object` widget (this is the default value for
+`widget`).
+
+### Additional list widget options
+
+- `types`: a nested list of object widgets. All widgets must be of type `object`. Every object widget may define different set of fields. 
+- `typeKey`: the name of the field that will be added to every item in list representing the name of the object widget that item belongs to. Ignored if `types` is not defined. Default is `type`.
+
+### Example Configuration
+
+The example configuration below imagines a scenario where the editor can add two "types" of content,
+either a "carousel" or a "spotlight". Each type has a unique name and set of fields.
+
+```yaml
+- label: "Home Section"
+  name: "sections"
+  widget: "list"
+  types:
+    - label: "Carousel"
+      name: "carousel"
+      widget: object
+      fields:
+        - {label: Header, name: header, widget: string, default: "Image Gallery"}
+        - {label: Template, name: template, widget: string, default: "carousel.html"}
+        - label: Images
+          name: images
+          widget: list
+          field: {label: Image, name: image, widget: image}
+    - label: "Spotlight"
+      name: "spotlight"
+      widget: object
+      fields:
+        - {label: Header, name: header, widget: string, default: "Spotlight"}
+        - {label: Template, name: template, widget: string, default: "spotlight.html"}
+        - {label: Text, name: text, widget: text, default: "Hello World"}
+```
+
+### Example Output
+
+The output for the list widget will be an array of objects, and each object will have a `type` key
+with the name of the type used for the list item. The `type` key name can be customized via the
+`typeKey` property in the list configuration.
+
+If the above example configuration were used to create a carousel, a spotlight, and another
+carousel, the output could look like this:
+
+```yaml
+title: Home
+sections:
+  - type: carousel
+    header: Image Gallery
+    template: carousel.html
+    images:
+      - images/image01.png
+      - images/image02.png
+      - images/image03.png
+  - type: spotlight
+    header: Spotlight
+    template: spotlight.html
+    text: Hello World
+  - type: carousel
+    header: Image Gallery
+    template: carousel.html
+    images:
+      - images/image04.png
+      - images/image05.png
+      - images/image06.png
+```
+
 ## Custom Mount Element
 Netlify CMS always creates its own DOM element for mounting the application, which means it always takes over the entire page, and is generally inflexible if you're trying to do something creative, like injecting it into a shared context.