[Security] Add getting started example to README

Author: wouterj

README.md

@@ -3,8 +3,40 @@ Security Component - Core
 
 Security provides an infrastructure for sophisticated authorization systems,
 which makes it possible to easily separate the actual authorization logic from
-so called user providers that hold the users credentials. It is inspired by
-the Java Spring framework.
+so called user providers that hold the users credentials.
+
+Getting Started
+---------------
+
+```
+$ composer require symfony/security-core
+```
+
+```php
+use Symfony\Component\Security\Core\Authentication\AuthenticationTrustResolver;
+use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
+use Symfony\Component\Security\Core\Authorization\AccessDecisionManager;
+use Symfony\Component\Security\Core\Authorization\Voter\AuthenticatedVoter;
+use Symfony\Component\Security\Core\Authorization\Voter\RoleVoter;
+use Symfony\Component\Security\Core\Authorization\Voter\RoleHierarchyVoter;
+use Symfony\Component\Security\Core\Exception\AccessDeniedException;
+use Symfony\Component\Security\Core\Role\RoleHierarchy;
+
+$accessDecisionManager = new AccessDecisionManager([
+    new AuthenticatedVoter(new AuthenticationTrustResolver()),
+    new RoleVoter(),
+    new RoleHierarchyVoter(new RoleHierarchy([
+        'ROLE_ADMIN' => ['ROLE_USER'],
+    ]))
+]);
+
+$user = new \App\Entity\User(...);
+$token = new UsernamePasswordToken($user, 'main', $user->getRoles());
+
+if (!$accessDecisionManager->decide($token, ['ROLE_ADMIN'])) {
+    throw new AccessDeniedException();
+}
+```
 
 Resources
 ---------