Docs on using behaviors

docs/guide/behaviors.md

 Behaviors
 =========
 
-TDB
+A behavior (also knows as mixin) can be used to enhance the functionality of an existing component without modifying its
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+code. In particular, it can "inject" its own methods and properties into the component and make them directly accessible
+via the component. It can also respond to the events triggered in the component and thus intercept the normal
+code execution. Unlike PHP traits, behaviors could be attached to classes at runtime.
+
+Using behaviors
+---------------
+
+Behavior can be attached to any class that extends from `Component`. In order to do so you need to implement `behaviors`
+method. Yii provides `AutoTimestamp` behavior that handles updating timestamp fields on saving active record model.
+
+```php
+class User extends ActiveRecord
+{
+	// ...
+
+	public function behaviors()
+	{
+		return [
+			'timestamp' => [
+				'class' => 'yii\behaviors\AutoTimestamp',
+				'attributes' => [
+					ActiveRecord::EVENT_BEFORE_INSERT => ['create_time', 'update_time'],
+					ActiveRecord::EVENT_BEFORE_UPDATE => 'update_time',
+				],
+			],
+		];
+	}
+}
+```
+
+In the above `class` value is a string containing fully qualified behavior class name. All the other key-values are
+assigned to corresponding properties of the class.
+
+Creating your own behaviors
+---------------------------
+
+TBD
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