[Joy] Update read.me content (#33643)

packages/mui-joy/README.md

@@ -1,3 +1,12 @@
 # @mui/joy
 
-[WIP] Joy design system — React components
+Joy UI is a library of beautifully designed React UI components.
+
+It features foundational components such as the ones you'd find in Material UI and it comes with a beautifully designed default theme so you can rapidly start your own design system. You should see Joy UI as a starting point. It comes with a lot of customization features so you match it to your desired look and feel.
+
+> **Warning**
+> As of today, Joy UI is in active development, with an alpha version soon to be released.
+> We're adding new components and features regularly, and you're welcome to contribute!
+> Look for the `package: joy` label on open issues and pull requests in the mui/material-ui repository on GitHub to see what other community members are working on, and feel free to submit your own.
+
+Get to know more at [Joy UI's overview page](https://mui.com/joy-ui/getting-started/overview/).