Details of paginators for customer data providers

This explains the need for paginators when using custom data providers,
and the available paginators.

Author: toby-griffiths

core/pagination.md

@@ -542,3 +542,15 @@ class BookRepository extends ServiceEntityRepository
     }
 }
 ```
+
+## Pagination for Customer Data Providers
+
+If you're using a customer Data Providers, and not the Doctrine ORM, ODM or
+ElasticSearch provides, then if you want your results to be paginated then
+you'll need to return an instance of a
+`\ApiPlatform\Core\DataProvider\PartialPaginatorInterface` or
+`\ApiPlatform\Core\DataProvider\PaginatorInterface`.  A few existing classes are
+provided to get you started with these…
+
+* `\ApiPlatform\Core\DataProvider\ArrayPaginator`
+* `\ApiPlatform\Core\DataProvider\TraversablePaginator`