Merge pull request #7799 from maxxer/explain_message_extract

Proposed documentation for i18n 'Using the message command'

docs/guide/tutorial-i18n.md

@@ -481,8 +481,28 @@ If [[yii\i18n\MissingTranslationEvent::translatedMessage]] is set by the event h
 
 ### Using the `message` command <a name="message-command"></a>
 
-TDB
+Translations can be stored in [[yii\i18n\PhpMessageSource|php files]], [[yii\i18n\GettextMessageSource|.po files] or to [[yii\i18n\DbMessageSource|database]]. See specific classes for additional options.
 
+First of all you need to create a config file. Decide where you want to store it and then issue the command 
+
+```bash
+./yii message/config path/to/config.php
+```
+
+Open the created file and adjust the parameters to fit your needs. Pay special attention to:
+
+* `languages`: an array representing what languages your app should be translated to;
+* `messagePath`: path where to store message files, which should match the `i18n`'s `basePath` parameter stated in config.
+
+> Note that aliases are not supported here, they must be real path relative to the config file location
+
+Once you're done with the config file you can finally extract your messages with the command
+
+```bash
+./yii message path/to/config.php
+```
+
+You will then find your files (if you've choosen file based translations) in your `messagePath` directory.
 
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