Update README with a demo video

Added a demo video to the README.

Author: level09

README.md

@@ -8,6 +8,14 @@ Multi-tenant workspaces, Stripe billing, OAuth, and team collaboration out of th
 
 **[Documentation](https://level09.github.io/readykit/)** · **[Live Demo](https://try.readykit.dev)**
 
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 ## What's Included