Update README with example data prop.

Author: kentwalters

README.md

@@ -18,8 +18,12 @@ $ yarn add react-retool
 import Retool from 'react-retool';
 
 function App() {
+  const sample = { name: 'Sample data' }
   return (
-    <Retool url="https://retoolin.tryretool.com/embedded/public/f7607e1f-670a-4ebf-9a09-be54cf17181e"/>
+    <Retool 
+      url="https://retoolin.tryretool.com/embedded/public/f7607e1f-670a-4ebf-9a09-be54cf17181e"
+      data={sample}
+    />
   );
 }