chore: Add SPDX header

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

docs/implement-import.md

@@ -1,3 +1,7 @@
+<!--
+  - SPDX-FileCopyrightText: 2021 Nextcloud GmbH and Nextcloud contributors
+  - SPDX-License-Identifier: AGPL-3.0-or-later
+-->
 ## Implement import
 
 * Create a new importer class extending `ABoardImportService`
@@ -29,4 +33,4 @@ $dispatcher->registerEventListener(
     YourCustomImporterListener::class
 );
 ```
-* Use the `lib/Service/Importer/Systems/TrelloJsonService.php` class as inspiration
+* Use the `lib/Service/Importer/Systems/TrelloJsonService.php` class as inspiration