Added assets section to the guide

docs/guide/assets.md

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+Managing assets
+===============
+
+An asset in Yii is a file that is included into the page. It could be CSS, JavaScript or
+any other file. Framework provides many ways to work with assets from basics such as adding `<script src="` tag
+for a file that is [handled by View](view.md) section to advanced usage such as pusblishing files that are not
+under webserve document root, resolving JavaScript dependencies or minifying CSS.
+
+Declaring asset bundle
+----------------------
+
+In order to publish some assets you should declare an asset bundle first. The bundle defines a set of asset files or
+directories to be published and their dependencies on other asset bundles.
+
+Both basic and advanced application templates contain `AppAsset` asset bundle class that defines assets required
+applicationwide. Let's review basic application asset bundle class:
+
+```php
+class AppAsset extends AssetBundle
+{
+	public $basePath = '@webroot';
+	public $baseUrl = '@web';
+	public $css = [
+		'css/site.css',
+	];
+	public $js = [
+	];
+	public $depends = [
+		'yii\web\YiiAsset',
+		'yii\bootstrap\BootstrapAsset',
+	];
+}
+```
+
+In the above `$basePath` specifies web-accessible directory assets are served from. It is a base for relative
+`$css` and `$js` paths i.e. `@webroot/css/site.css` for `css/site.css`. Here `@webroot` is an alias that points to
+application's `web` directory.
+
+`$baseUrl` is used to specify base URL for the same relative `$css` and `$js` i.e. `@web/css/site.css` where `@web`
+is an alias that corresponds to your website base URL such as `http://example.com/`.
+
+In case you have asset files under non web accessible directory, that is the case for any extension, you need
+to additionally specify `$sourcePath`. Files will be copied or symlinked from source bath to base path prior to being
+registered. In case source path is used `baseUrl` is generated automatically at the time of publising asset bundle.
+
+Dependencies on other asset bundles are specified via `$depends` property. It is an array that contains fully qualified
+names of bundle classes that should be published in order for this bundle to work properly.
+
+Here `yii\web\YiiAsset` adds Yii's JavaScript library while `yii\bootstrap\BootstrapAsset` includes
+[Bootstrap](http://getbootstrap.com) frontend framework.
+
+Asset bundles are regular classes so if you need to define another one, just create alike class with unique name. This
+class can be placed anywhere but the convention for it is to be under `assets` directory of the applicaiton.
+
+Registering asset bundle
+------------------------
+
+Asset bundle classes are typically registered in views or, if it's main application asset, in layout. Doing it is
+as simple as:
+
+```php
+use app\assets\AppAsset;
+AppAsset::register($this);
+```
+
+Since we're in a view context `$this` refers to `View` class.
+
+Overriding asset bundles
+------------------------
+
+Sometimes you need to override some asset bundles application wide. A good example is loading jQuery from CDN instead
+of your own server. In order to do it we need to configure `assetManager` application component via config file. In case
+of basic application it is `config/web.php`:
+
+```php
+return [
+    // ...
+    'components' => [
+        'assetManager' => [
+            'bundles' => [
+                'yii\web\JqueryAsset' => [
+                     'sourcePath' => null,
+                     'js' => ['//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js']
+                ],
+            ],
+        ],
+    ],
+];
+```
+
+In the above we're adding asset bundle definitions to `bunldes` property of asset manager. Keys there are fully
+qualified class paths to asset bundle classes we want to override while values are key-value arrays of class properties
+and corresponding values to set.
+
+Setting `sourcePath` to `null` tells asset manager not to copy anything while `js` overrides local files with a link
+to CDN.
+
+Enabling symlinks
+-----------------
+
+Asset manager is able to use symlinks instead of copying files. It is turned off by default since symlinks are often
+disabled on shared hosting. If your hosting environment supports symlinks you certainly should enable the feature via
+application config:
+
+```php
+return [
+    // ...
+    'components' => [
+        'assetManager' => [
+            'linkAssets' => true,
+        ],
+    ],
+];
+```
+
+There are two main benefits in enabling it. First it is faster since no copying is required and second is that assets
+will always be up to date with source files.