Add info about setting locale folder path.

Refs #9074

src/I18n/README.md

@@ -23,6 +23,16 @@ use Cake\I18n\I18n;
 I18n::locale('en_US');
 ```
 
+### Setting path to folder containing po files.
+
+```php
+use Cake\Core\Configure;
+
+Configure::write('App.paths.locales', ['/path/with/trailing/slash/']);
+
+Please refer to the [CakePHP Manual](http://book.cakephp.org/3.0/en/core-libraries/internationalization-and-localization.html#language-files) for details
+about expected folder structure and file naming.
+
 ### Translating a Message
 
 ```php