sandbox/README: add link to live deployment

The sandbox is deployed to GitHub pages automatically.

Author: benma

sandbox/README.md

@@ -1,5 +1,7 @@
 # Demo WebApp project
 
+A live deployment of this demo can be found here: https://digitalbitbox.github.io/bitbox-api-rs/.
+
 This folder contains a React project showcasing the TypeScript API. It has the bitbox-api library in
 `../pkg` as a dependency. See how to build this dependency locally at the [main
 README](../.github/README.md).