Added $translator->addLoader()

For the example to work it needs the line with $translator->addLoader().
Fixed it for request at https://github.com/symfony/symfony/issues/39854#issuecomment-762283989

src/Symfony/Component/Translation/README.md

@@ -12,8 +12,10 @@ $ composer require symfony/translation
 
 ```php
 use Symfony\Component\Translation\Translator;
+use Symfony\Component\Translation\Loader\ArrayLoader;
 
 $translator = new Translator('fr_FR');
+$translator->addLoader('array', new ArrayLoader());
 $translator->addResource('array', [
     'Hello World!' => 'Bonjour !',
 ], 'fr_FR');