Document how to load the SPA SDK in index.html

[Widcket]

• [X] All new/changed/fixed functionality is covered by tests (or N/A)
• [X] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR documents in the README of the app-facing package how to load the SPA SDK by adding a <script> tag to the index.html file.