"Using 3rd-Party Libraries" guide added

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

+Using 3rd-Party Libraries
+=========================
+
+Yii is carefully designed so that third-party libraries can be
+easily integrated to further extend Yii's functionalities.
+
+TODO: namespaces and composer explanations
+
+Using Yii in 3rd-Party Systems
+------------------------------
+
+Yii can also be used as a self-contained library to support developing and enhancing
+existing 3rd-party systems, such as WordPress, Joomla, etc. To do so, include
+the following code in the bootstrap code of the 3rd-party system:
+
+```php
+$yiiConfig = require(__DIR__ . '/../config/yii/web.php');
+new yii\web\Application($yiiConfig); // No 'run()' invocation!
+```
+
+The above code is very similar to the bootstrap code used by a typical Yii application
+except one thing: it does not call the `run()` method after creating the Web application
+instance.
+
+Now we can use most features offered by Yii when developing 3rd-party enhancements. For example,
+we can use `Yii::$app` to access the application instance; we can use the database features
+such as ActiveRecord; we can use the model and validation feature; and so on.
+
+Using Yii2 with Yii1
+--------------------
+
+Yii2 can be used along with Yii1 at the same project.
+Since Yii2 uses namespaced class names they will not conflict with any class from Yii1.
+However there is single class, which name is used both in Yii1 and Yii2, it named 'Yii'.
+In order to use both Yii1 and Yii2 you need to resolve this collition.
+To do so you need to define your own 'Yii' class, which will combine content of 'Yii' from 1.x
+and 'Yii' from 2.x.
+
+Start from defining your own descendant of [[\yii\BaseYii]]:
+
+```php
+$yii2path = '/path/to/yii2';
+require($yii2path . '/BaseYii.php');
+
+class Yii extends \yii\BaseYii
+{
+}
+
+Yii::$classMap = include($yii2path . '/classes.php');
+```
+
+Now we have a class, which suites Yii2, but causes fatal errors for Yii1.
+So, first of all, we need to include [[\YiiBase]] of Yii1 source code to our 'Yii' class
+definition file:
+
+```php
+$yii2path = '/path/to/yii2';
+require($yii2path . '/BaseYii.php'); // Yii 2.x
+$yii1path = '/path/to/yii1';
+require($yii1path . '/YiiBase.php'); // Yii 1.x
+
+class Yii extends \yii\BaseYii
+{
+}
+
+Yii::$classMap = include($yii2path . '/classes.php');
+```
+
+Using this, defines all necessary constants and autoloader of Yii1.
+Now we need to add all fields and methods from [[\YiiBase]] of Yii1 to our 'Yii' class.
+Unfortunally, there is no way to do so but copy-paste:
+
+```php
+$yii2path = '/path/to/yii2';
+require($yii2path . '/BaseYii.php');
+$yii1path = '/path/to/yii1';
+require($yii1path . '/YiiBase.php');
+
+class Yii extends \yii\BaseYii
+{
+	public static $classMap = [];
+	public static $enableIncludePath = true;
+	private static $_aliases = ['system'=>YII_PATH,'zii'=>YII_ZII_PATH]; // alias => path
+	private static $_imports = [];										// alias => class name or directory
+	private static $_includePaths;										// list of include paths
+	private static $_app;
+	private static $_logger;
+
+	public static function getVersion()
+	{
+		return '1.1.15-dev';
+	}
+
+	public static function createWebApplication($config=null)
+	{
+		return self::createApplication('CWebApplication',$config);
+	}
+
+	public static function app()
+	{
+		return self::$_app;
+	}
+
+	// Rest of \YiiBase internal code placed here
+	...
+}
+
+Yii::$classMap = include($yii2path . '/classes.php');
+Yii::registerAutoloader(['Yii', 'autoload']); // Register Yii2 autoloader via Yii1
+```
+
+Now we have 'Yii' class, which suites both Yii 1.x and Yii 2.x.
+So bootstrap code used by your application will looks like following:
+
+```php
+require(__DIR__ . '/../components/my/Yii.php'); // include created 'Yii' class
+
+$yii2Config = require(__DIR__ . '/../config/yii2/web.php');
+new yii\web\Application($yii2Config); // create Yii 2.x application
+
+$yii1Config = require(__DIR__ . '/../config/yii1/main.php');
+Yii::createWebApplication($yii1Config)->run(); // create Yii 1.x application
+```
+
+Then in any part of your program ```Yii::$app``` refers to Yii 2.x application,
+while ```Yii::app()``` refers to Yii 1.x application:
+
+```php
+echo get_class(Yii::app()); // outputs 'CWebApplication'
+echo get_class(Yii::$app); // outputs 'yii\web\Application'
+```
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