Update README.md

Add Plugin part on Readme

Author: lam0819

README.md

@@ -24,6 +24,15 @@ composer require solution-forest/filament-simplelightbox
 
 ## Usage
 
+```php
+public function panel(Panel $panel): Panel
+{
+    return $panel
+        // ...
+        ->plugin(SimpleLightBoxPlugin::make())
+}
+```
+
 ```php
 Tables\Columns\ImageColumn::make('image')
                     ->simpleLightbox()