docs: update supported symfony versions

Author: evansims

README.md

@@ -35,8 +35,8 @@ The following `psr/container-implementation` implementations are discovered and
 -   [league/container](https://github.com/thephpleague/container) ^3.0 | ^4.0
 -   [php-di/php-di](https://github.com/PHP-DI/PHP-DI) ^5.4.2 | ^6.0 | ^7.0
 -   [silverstripe/framework](https://github.com/silverstripe/silverstripe-framework) ^4.0
--   [symfony/dependency-injection](https://github.com/symfony/dependency-injection) ^3.3 | ^4.0 | ^5.0 | ^6.0
--   [symfony/symfony](https://github.com/symfony/symfony) ^3.3 | ^4.0 | ^5.0 | ^6.0
+-   [symfony/dependency-injection](https://github.com/symfony/dependency-injection) ^3.3 | ^4.0 | ^5.0 | ^6.0 | ^7.0
+-   [symfony/symfony](https://github.com/symfony/symfony) ^3.3 | ^4.0 | ^5.0 | ^6.0 | ^7.0
 -   [yiisoft/di](https://github.com/yiisoft/di) ^1.0
 
 The following mock implementations are also available: