Update readme to include class import statement

Author: timciep

README.md

@@ -20,6 +20,8 @@ composer require idf/nova-html-card
 [Register your new card](https://nova.laravel.com/docs/2.0/customization/cards.html#registering-cards).
 
 ```php
+use IDF\HtmlCard\HtmlCard;
+
 public function cards()
 {
     return [ 
@@ -70,4 +72,4 @@ npm run watch
 
 ## License
 
-The MIT License (MIT). Please see [License File](LICENSE) for more information.
+The MIT License (MIT). Please see [License File](LICENSE) for more information.