added README file

extensions/yii/imagine/README.md

+Image Extension for Yii 2
+==============================
+
+This extension adds most common image functions and also acts as a wrapper to [Imagine](http://imagine.readthedocs.org/)
+image manipulation library.
+
+Installation
+------------
+
+The preferred way to install this extension is through [composer](http://getcomposer.org/download/).
+
+Either run
+
+```
+php composer.phar require yiisoft/yii2-imagine "*"
+```
+
+or add
+
+```json
+"yiisoft/yii2-imagine": "*"
+```
+
+to the `require` section of your composer.json.
+
+
+Usage & Documentation
+---------------------
+
+This extension is a wrapper to the [Imagine](http://imagine.readthedocs.org/) and also adds the most common methods
+used for Image manipulation.
+
+To use this extension, you can use it in to ways, whether you configure it on your application file or you use it
+directly.
+
+The following shows how to use it via application configuration file:  
+
+```
+// configuring on your application configuration file
+'components' => [
+    'image' => [
+        'class' => 'yii\imagine\Image',
+        'driver' => \yii\imagine\Image::DRIVER_GD2,
+    ]
+    ...
+]
+
+// Once configured you can access to the extension like this:
+$img = Yii::$app->image->thumb('path/to/image.jpg', 120, 120);
+
+```
+
+This is how to use it directly:
+
+```
+use yii\imagine\Image;
+
+$image = new Image();
+$img = $image->thumb('path/to/image.jpg', 120, 120);
+```
+**About the methods**  
+Each method returns an instance to `\Imagine\Image\ManipulatorInterface`, that means that you can easily make use of the methods included in the `Imagine` library: 
+
+```
+// frame, rotate and save an image 
+
+Yii::$app->image->frame('path/to/image.jpg', 5, '666', 0)
+    ->rotate(-8)
+    ->save('path/to/destination/image.jpg', ['quality' => 50]);
+```
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