Documentation regarding PSR2 vs PSR12 standard

[aurelijusb]

Symfony has its own coding standard, that is based on PSR-2. Despite the fact, that PSR-2 is deprecated.

Documentation:

• https://www.php-fig.org/psr/psr-2/
• https://github.com/symfony/symfony/issues/33324
• https://symfony.com/doc/current/contributing/code/standards.html
• https://blog.jetbrains.com/phpstorm/2019/10/phpstorm-2019-3-eap-5/