Update docs to include FileFormService

Author: davidjamesstone

docs/features/code-based/CUSTOM_SERVICES.md

@@ -44,3 +44,55 @@ This method is invoked for every page request.
 
 Only when the `formMetadata` indicates that the definition has changed is a call to `getFormDefinition` is made.
 The response from this can be quite big as it contains the entire form definition.
+
+## Loading forms from files
+
+To create a `formsService` from form config files that live on disk, you can use the `FileFormService` class.
+Form definition config files can be either `.json` or `.yaml`.
+
+Once created and files have been loaded using the `addForm` method,
+call the `toFormsService` method to return a `FormService` compliant interface which can be passed in to the `services` setting of the [plugin options](/forms-engine-plugin/PLUGIN_OPTIONS.md).
+
+```javascript
+import { FileFormService } from '@defra/forms-engine-plugin/file-form-service.js'
+
+// Create shared form metadata
+const now = new Date()
+const user = { id: 'user', displayName: 'Username' }
+const author = { createdAt: now, createdBy: user, updatedAt: now, updatedBy: user }
+const metadata = {
+  organisation: 'Defra',
+  teamName: 'Team name',
+  teamEmail: 'team@defra.gov.uk',
+  submissionGuidance: "Thanks for your submission, we'll be in touch",
+  notificationEmail: 'email@domain.com',
+  ...author,
+  live: author
+}
+
+// Instantiate the file loader form service
+const loader = new FileFormService()
+
+// Add a Json form
+await loader.addForm(
+  'src/definitions/example-form.json', {
+    ...metadata,
+    id: '95e92559-968d-44ae-8666-2b1ad3dffd31',
+    title: 'Example Json',
+    slug: 'example-json'
+  }
+)
+
+// Add a Yaml form
+await loader.addForm(
+  'src/definitions/example-form.json', {
+    ...metadata,
+    id: '641aeafd-13dd-40fa-9186-001703800efb',
+    title: 'Example Yaml',
+    slug: 'example-yaml'
+  }
+)
+
+// Get the forms service
+const formsService = loader.toFormsService()
+```