docs about default translation added

docs/guide/i18n.md

@@ -78,7 +78,7 @@ Yii tries to load appropriate translation from one of the message sources define
 ```
 
 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`.
+handling everything that begins with `app`. You can also specify default translation, for more info see [this](specifying-default-translation) example.
 
 `class` defines which message source is used. The following message sources are available:
 
@@ -287,6 +287,43 @@ extension.
 Examples
 --------
 
+###Specifying default translation
+
+You can specify default translation, that will be used as a fallback for categories, that don't match any other translation.
+This translation should be marked with `*`. For example do this in your config file (for the `yii2-basic` application it will be `web.php`):
+
+```php
+//configure i18n component
+
+'i18n' => [
+	'translations' => [
+		'*' => [
+			'class' => 'yii\i18n\PhpMessageSource'
+		],
+	],
+],
+```
+
+After that you can start using your categories without specifying them in i18n translations. This is more familiar as for Yii1. Now you can create
+files under default category `basePath` and simply use them as usual, for example:
+
+1. create your category file under messages `basePath`, according default translation. Since we are not overriding `basePath` property of
+[`PhpMessageSource`](https://github.com/yiisoft/yii2/blob/master/framework/i18n/PhpMessageSource.php#L40) it would be `@app/messages`.
+
+```php
+# not_specified_category.php file under @app/messages directory.
+
+return [
+	'message from not specified category' => 'сообщение из не указанной категории',
+];
+```
+
+2. Use your category as before.
+
+```php
+echo Yii::t('not_specified_category','message from not specified category');
+```
+
 ###Translating module messages
 
 If you want to translate messages for a module and avoid using a single translation file for all messages, you can make it like the following: