Add custom file name config information

[mkinternet]

https://github.com/octobercms/docs/blob/develop/1.x/plugin/settings.md

/plugins/author/myplugin/config/custom.php
becomes
Config::get('Acme.Demo::custom.maxItems');

[daftspunk]

Thanks. Added this to docs

---

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);