linked guide on 3rd party systems

docs/guide/index.md

@@ -48,6 +48,7 @@ Extensions and 3rd party libraries
 - [Composer](composer.md) - How to manage applications dependencies via composer
 - [Extending Yii](extensions.md)
 - [Template engines](template.md) - Using template engines such as Smarty or Twig
+- [Using Yii together with 3rd-Party Systems](using-3rd-party-libraries.md) - Using Yii in 3rd-Party Systems and using Yii 1 and 2 together
 
 Security and access control
 ===========================

docs/guide/upgrade-from-v1.md

@@ -523,3 +523,8 @@ up to date updating it semi-automatically and manages autoloading for third part
 these are using.
 
 In order to learn more refer to [composer](composer.md) and [installation](installation.md) sections of the guide.
+
+Using Yii 1.1 and 2.x together
+------------------------------
+
+Check the guide on [using Yii together with 3rd-Party Systems](using-3rd-party-libraries.md) on this topic.

docs/guide/using-3rd-party-libraries.md

@@ -36,6 +36,15 @@ In order to use both Yii1 and Yii2 you need to resolve this collision.
 To do so you need to define your own 'Yii' class, which will combine content of 'Yii' from 1.x
 and 'Yii' from 2.x.
 
+When using composer you add the following to your composer.json in order to add both versions of yii to your project:
+
+```json
+"require": {
+        "yiisoft/yii": "*",
+        "yiisoft/yii2": "*",
+    },
+```
+
 Start from defining your own descendant of [[\yii\BaseYii]]:
 
 ```php