Merge branch '2.8' into 3.2

* 2.8:
  Revamped the README file
  Fix missing namespace in AddConstraintValidatorPassTest
  [ExpressionLanguage] Registering functions after calling evaluate(), compile() or parse() is not supported

Author: fabpot

Tests/DependencyInjection/Compiler/AddConstraintValidatorsPassTest.php

@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+namespace Symfony\Bundle\FrameworkBundle\Tests\DependencyInjection\Compiler;
+
 use PHPUnit\Framework\TestCase;
 use Symfony\Bundle\FrameworkBundle\DependencyInjection\Compiler\AddConstraintValidatorsPass;