Updated README

[ci skip]

Author: tobihagemann

README.md

@@ -18,6 +18,8 @@ If you are building with Xcode, create a `.cloud-access-secrets.sh` file in the
 
 ```sh
 #!/bin/sh
+export BOX_CLIENT_ID=...
+export BOX_CLIENT_SECRET=...
 export DROPBOX_APP_KEY=...
 export GOOGLE_DRIVE_CLIENT_ID=...
 export GOOGLE_DRIVE_REDIRECT_URL_SCHEME=...