Add configuration options doc (#885)

* Add collections options doc
* Move Editorial Workflow page; transfer content to config and architecture docs

website/site/content/docs/architecture.md

@@ -56,3 +56,21 @@ For either updating an existing entry or creating a new one, the `EntryEditor` i
 #### Widget components implementation
 The control component receives one (1) callback as a prop: `onChange`.
 * onChange (required): Should be called when the users changes the current value. It will ultimately end up updating the EntryDraft object in the Redux Store, thus updating the preview component.
+* onAddAsset & onRemoveAsset (optionals): If the field accepts file uploads for media (images, for example), these callbacks should be invoked with a `AssetProxy` value object. `onAddAsset` will get the current media stored in the Redux state tree while `onRemoveAsset` will remove it. AssetProxy objects are stored in the `Medias` object and referenced in the `EntryDraft` object on the state tree.
+
+Both control and preview widgets receive a `getAsset` selector via props. Displaying the media (or its URI) for the user should always be done via `getAsset`, as it returns an AssetProxy that can return the correct value for both medias already persisted on the server and cached media not yet uploaded.
+
+The actual persistence of the content and medias inserted into the control component is delegated to the backend implementation. The backend will be called with the updated values and a list of assetProxy objects for each field of the entry, and should return a promise that can resolve into the persisted entry object and the list of the persisted media URIs.
+
+
+## Editorial Workflow implementation
+
+Instead of adding logic to `CollectionPage` and `EntryPage`, the Editorial Workflow is implemented as Higher Order Components, adding UI and dispatching additional actions.
+
+Furthermore, all editorial workflow state is managed in Redux - there's an `actions/editorialWorkflow.js` file and a `reducers/editorialWorkflow.js` file.
+
+### About metadata
+
+Netlify CMS embraces the idea of Git-as-backend for storing metadata. The first time it runs with the editorial_workflow setup, it creates a new ref called `meta/_netlify_cms`, pointing to an empty, orphan tree.
+
+Actual data are stored in individual `json` files committed to this tree.