docs(readme): add NgModule to import example

Author: nivekcode

README.md

@@ -30,7 +30,10 @@ Import the ```NgsgModule``` in your ```AppModule```.
 ```
   import {NgsgModule} from 'ng-sortgrid'
   ...
-  imports: [BrowserModule, NgsgModule],
+  @NgModule({
+    imports: [BrowserModule, NgsgModule],
+    //...
+  })  
   ...
 ```