chore: Add SPDX header

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

appinfo/info.xml

@@ -1,4 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
+<!--
+  - SPDX-FileCopyrightText: 2016 Nextcloud GmbH and Nextcloud contributors
+  - SPDX-License-Identifier: AGPL-3.0-or-later
+-->
 <info xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="https://apps.nextcloud.com/schema/apps/info.xsd">
     <id>deck</id>
     <name>Deck</name>