docs: Add reuse badge

Signed-off-by: Andy Scherzinger <[email protected]>

Author: AndyScherzinger

README.md

@@ -4,6 +4,8 @@
 -->
 # End-to-End Encryption App
 
+[![REUSE status](https://api.reuse.software/badge/github.com/nextcloud/end_to_end_encryption)](https://api.reuse.software/info/github.com/nextcloud/end_to_end_encryption)
+
 This app provides all the necessary APIs to implement End-to-End encryption
 on the client side. Additionally it makes sure that End-to-End encrypted
 files are not accessible with the web interface and other WebDAV clients.