Add note about SSE

Signed-off-by: Julius Härtl <jus@bitgrid.net>

README.md

@@ -6,4 +6,8 @@ files are not accessible with the web interface and other WebDAV clients.
 
 Here you can find the [API documentation](https://github.com/nextcloud/end_to_end_encryption/blob/master/doc/api.md).
 
+## Limitation
+
+- E2EE is currently not compatible to be used together with server-side encryption
+
 ![](doc/screenshots/e2ee-files-listing.png)