Add support for Drupal hash salt in separate file

[cristiroma]

Best practice recommends (also found in default.settings.php) to store the Drupal password hash salt in a separate text file outside project repository and load it using file_get_contents. Therefore we can add an example for new project setups:

1. Add the configuration in example.settings.local.php

$config['hash_salt'] = file_get_contents('/var/www/html/config/example.com/drupal-hash-salt.txt`);

2. Add instructions in README.md

[tv889]

Logged ticket https://helpdesk.eaudeweb.ro/issues/17031

[stefanbutura]

Hash salt setup is in README, the file is read from settings.php.