Added guide for contributing message translations

docs/internals/translations.md

+Translation workflow
+====================
+
+Yii is translated in many languages in order to be useful for international applications and developers. Two main areas
+where contribution is very welcome are documentation and framework messages.
+
+Framework messages
+------------------
+
+Framework has two types of messsages: exceptions that are intended to developer and are never translated and messages
+that are actually visible to end user such as validation errors.
+
+In order to start with message translation:
+
+1. Check `framework/yii/messages/config.php` and make sure your language is listed in `languages`.
+2. Go to `framework/yii` and run `yii message/extract messages/config.php`.
+3. Translate messages in `framework/yii/messages/your_language/yii.php`. Make sure file is saved using UTF-8 encoding.
+4. Make a pull request.
+
+In order to keep your translation up to date you may run `yii message/extract messages/config.php` again. It will
+automatically re-extract messages keeping unchanged ones intact.
+
+In the translation file each array element represents the translation (value) of a message (key). If the value is empty,
+the message is considered as not translated. Messages that no longer need translation will have their translations
+enclosed between a pair of '@@' marks. Message string can be used with plural forms format. Check [i18n section
+of the guide](i18n.md) for details.