[Doc] Use Markdown syntax highlighting

Author: lologhi

README.md

@@ -12,12 +12,14 @@ The component provides "validation constraints", which are simple objects
 containing the rules for the validation. Let's validate a simple string
 as an example:
 
-    use Symfony\Component\Validator\Validation;
-    use Symfony\Component\Validator\Constraints\Length;
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints\Length;
 
-    $validator = Validation::createValidator();
+$validator = Validation::createValidator();
 
-    $violations = $validator->validateValue('Bernhard', new Length(array('min' => 10)));
+$violations = $validator->validateValue('Bernhard', new Length(array('min' => 10)));
+```
 
 This validation will fail because the given string is shorter than ten
 characters. The precise errors, here called "constraint violations",  are
@@ -26,24 +28,26 @@ If the violation list is empty, validation succeeded.
 
 Validation of arrays is possible using the `Collection` constraint:
 
-    use Symfony\Component\Validator\Validation;
-    use Symfony\Component\Validator\Constraints as Assert;
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints as Assert;
 
-    $validator = Validation::createValidator();
+$validator = Validation::createValidator();
 
-    $constraint = new Assert\Collection(array(
-        'name' => new Assert\Collection(array(
-            'first_name' => new Assert\Length(array('min' => 101)),
-            'last_name'  => new Assert\Length(array('min' => 1)),
-        )),
-        'email'    => new Assert\Email(),
-        'simple'   => new Assert\Length(array('min' => 102)),
-        'gender'   => new Assert\Choice(array(3, 4)),
-        'file'     => new Assert\File(),
-        'password' => new Assert\Length(array('min' => 60)),
-    ));
+$constraint = new Assert\Collection(array(
+    'name' => new Assert\Collection(array(
+        'first_name' => new Assert\Length(array('min' => 101)),
+        'last_name'  => new Assert\Length(array('min' => 1)),
+    )),
+    'email'    => new Assert\Email(),
+    'simple'   => new Assert\Length(array('min' => 102)),
+    'gender'   => new Assert\Choice(array(3, 4)),
+    'file'     => new Assert\File(),
+    'password' => new Assert\Length(array('min' => 60)),
+));
 
-    $violations = $validator->validateValue($input, $constraint);
+$violations = $validator->validateValue($input, $constraint);
+```
 
 Again, the validator returns the list of violations.
 
@@ -52,45 +56,47 @@ a mapping you can put constraints onto properties and objects of classes.
 Whenever an object of this class is validated, its properties and
 method results are matched against the constraints.
 
-    use Symfony\Component\Validator\Validation;
-    use Symfony\Component\Validator\Constraints as Assert;
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints as Assert;
+
+class User
+{
+    /**
+     * @Assert\Length(min = 3)
+     * @Assert\NotBlank
+     */
+    private $name;
+
+    /**
+     * @Assert\Email
+     * @Assert\NotBlank
+     */
+    private $email;
+
+    public function __construct($name, $email)
+    {
+        $this->name = $name;
+        $this->email = $email;
+    }
 
-    class User
+    /**
+     * @Assert\True(message = "The user should have a Google Mail account")
+     */
+    public function isGmailUser()
     {
-        /**
-         * @Assert\Length(min = 3)
-         * @Assert\NotBlank
-         */
-        private $name;
-
-        /**
-         * @Assert\Email
-         * @Assert\NotBlank
-         */
-        private $email;
-
-        public function __construct($name, $email)
-        {
-            $this->name = $name;
-            $this->email = $email;
-        }
-
-        /**
-         * @Assert\True(message = "The user should have a Google Mail account")
-         */
-        public function isGmailUser()
-        {
-            return false !== strpos($this->email, '@gmail.com');
-        }
+        return false !== strpos($this->email, '@gmail.com');
     }
+}
 
-    $validator = Validation::createValidatorBuilder()
-        ->enableAnnotationMapping()
-        ->getValidator();
+$validator = Validation::createValidatorBuilder()
+    ->enableAnnotationMapping()
+    ->getValidator();
 
-    $user = new User('John Doe', '[email protected]');
+$user = new User('John Doe', '[email protected]');
 
-    $violations = $validator->validate($user);
+$violations = $validator->validate($user);
+```
 
 This example uses the annotation support of Doctrine Common to
 map constraints to properties and methods. You can also map constraints