docs(dataproducter): Add how to resolve user properties to docs (#1292)

Author: Cryt1c

doc/producers/custom.md

@@ -136,4 +136,15 @@ we get a result like this :
 }
 ```
 
-(For this to actually work we would need to add resolvers to the User object to resolve the `id` and `name` properties).
+For this to actually work we would need to add resolvers to the User object to resolve the `id` and `name` properties like so:
+```php
+    $registry->addFieldResolver('User', 'id', $builder->callback(function ($account) {
+      /** @var \Drupal\Core\Session\AccountProxyInterface $account */
+      return $account->id();
+    }));
+
+    $registry->addFieldResolver('User', 'name', $builder->callback(function ($account) {
+      /** @var \Drupal\Core\Session\AccountProxyInterface $account */
+      return $account->getAccountName();
+    }));
+```