docs: Add reuse badge

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

README.md

@@ -4,6 +4,8 @@
 -->
 # Files viewer for nextcloud
 
+[![REUSE status](https://api.reuse.software/badge/github.com/nextcloud/viewer)](https://api.reuse.software/info/github.com/nextcloud/viewer)
+
 Show your latest holiday photos and videos like in the movies. Show a glimpse of your latest novel directly from your nextcloud. Choose the best GIF of your collection thanks to the direct view of your favorites files!
 
 ![viewer](https://github.com/raw/nextcloud/screenshots/master/apps/Viewer/viewer.png?v=2)