Add custom file name config information about docs

[daftspunk]

Thanks. Added this to docs

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You may also use a different filename for the configuration file and this affects the key name. For example, a configuration file named custom.php will prefix the key name with custom, using the following format: Acme.Demo::custom.maxItems. Example configuration file plugins/acme/demo/config/custom.php.

$maxItems = Config::get('acme.demo::custom.maxItems', 50);