Update README with code example

README.md

@@ -3,6 +3,14 @@
 Generate long-lived tokens from the CLI that can be used with the MapKit JS's [`authorizationCallback`](https://developer.apple.com/documentation/mapkitjs/creating_and_using_tokens_with_mapkit_js#3138220).
 
 > Setting `authorizationCallback` to a function that returns a token as a string is useful for local development, or if you want to use a long-lived token with MapKit JS. Sign a token locally on your development machine with an expiration, then use the token directly in your code.
+>
+> ```
+> mapkit.init({
+>    authorizationCallback(done) {
+>        done('your-generated-token-string-here');
+>    },
+> });
+> ```
 
 ### Features
 * Verifies if the token works against MapKit servers