Fixes #930: Added i18n docs about configuring message source

docs/guide/i18n.md

@@ -47,9 +47,46 @@ translation of the message from source language into target language. Message it
 echo \Yii::t('app', 'This is a string to translate!');
 ```
 
-Yii tries to load approprite translation from one of the message sources defined via `i18n` component configuration.
+Yii tries to load approprite translation from one of the message sources defined via `i18n` component configuration:
 
-TBD: https://github.com/yiisoft/yii2/issues/930
+```php
+'components' => array(
+	// ...
+	'i18n' => array(
+		'translations' => array(
+			'app*' => array(
+				'class' => 'yii\i18n\PhpMessageSource',
+				//'basePath' => '@app/messages',
+				//'sourceLanguage' => 'en_US',
+				'fileMap' => array(
+					'app' => 'app.php',
+					'app/error' => 'error.php',
+				),
+			),
+		),
+	),
+),
+```
+
+In the above `app*` is a pattern that specifies which categories are handled by the message source. In this case we're
+handling everything that begins with `app`.
+
+`class` defines which message source is used. There following message sources are available:
+
+- PhpMessageSource that uses PHP files.
+- GettextMessageSource that uses GNU Gettext MO or PO files.
+- DbMessageSource that uses database.
+
+`basePath` defines where to store messages for the currently used message source. In this case it's `messages` directory
+ in your application directory. In case of using database this option should be skipped.
+
+`sourceLanguage` defines which language is used in `\Yii::t` second argument. If not specified, application's source
+language is used.
+
+`fileMap` specifies how message categories specified in the first argument of `\Yii::t()` are mapped to files when
+`PhpMessageSource` is used. In the example we're defining two categories `app` and `app/error`.
+
+Instead of configuring `fileMap` you can rely on convention which is `messages/BasePath/LanguageID/CategoryName.php`.
 
 ### Named placeholders