docs: Add REUSE compliance status badge

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

README.md

@@ -4,6 +4,7 @@
  - SPDX-License-Identifier: AGPL-3.0-or-later
 -->
 # Nextcloud Server ☁
+[![REUSE status](https://api.reuse.software/badge/github.com/nextcloud/server)](https://api.reuse.software/info/github.com/nextcloud/server)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/nextcloud/server/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/nextcloud/server/?branch=master)
 [![codecov](https://codecov.io/gh/nextcloud/server/branch/master/graph/badge.svg)](https://codecov.io/gh/nextcloud/server)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/209/badge)](https://bestpractices.coreinfrastructure.org/projects/209)