Add PHP version check and some feedback to the user, list requirements in readme

The requirement for PHP 5.3.6 can only be seen in the composer file.
This isn't very handy for users that work with the `.phar` file. Running PHP-CS-Fixer with a lower version of PHP sometimes results in the cryptic `segmentation fault`. I think it will be more user friendly to tell them upfront it is not going to work.