Add variants callback syntax

packages/zero-runtime/README.md

@@ -16,7 +16,6 @@ npm install --save-dev @mui/zero-next-plugin
 npm install --save-dev @mui/zero-vite-plugin
 ```
 
-
 #### Next.js
 
 In your `next.config.js` file,
@@ -47,7 +46,6 @@ Setting `sourceMap` to true will take you to the style definition in the JS file
 
 #### Vite
 
-
 In your vite config file,
 
 1. Import the plugin
@@ -173,6 +171,16 @@ const Button = styled.button(() => ({
         // Styles to be applied when <Button size="small" color="primary" />
       },
     },
+    {
+      // If key value pair doesn't suffice, you can use the callback syntax
+      // to return `true` if you want the styles to be applied
+      props({ children }) {
+        return !!children;
+      },
+      style: {
+        // CSS
+      },
+    },
   ],
 }));
 ```