use public const declaration in class SystemConfiguration

[MaximGns]

In Phpro\APILogger\Config\SystemConfiguration replace:

const XML_LOG_FILE_NAME = 'system/api_log/file';
const XML_LOG_LEVEL = 'system/api_log/level';
const XML_ADD_SLASHES = 'system/api_log/add_slashes';

const LOG_DIR = 'var' . DIRECTORY_SEPARATOR . 'log' . DIRECTORY_SEPARATOR;

To

public const XML_LOG_FILE_NAME = 'system/api_log/file';
public const XML_LOG_LEVEL = 'system/api_log/level';
public const XML_ADD_SLASHES = 'system/api_log/add_slashes';

public const LOG_DIR = 'var' . DIRECTORY_SEPARATOR . 'log' . DIRECTORY_SEPARATOR;

And update the README.md documentation