docs on configuring Yii

docs/guide/configuration.md

+Configuration
+=============
+
+In Yii application and majority of components have sensible defaults so it's unlikely you spend lots of time configuring
+it. Still there are some mandatory options such as database connection you should set up.
+
+How application is configured depends on application template but there are some genral principles applying in any case.
+
+Configuring options in bootstrap file
+-------------------------------------
+
+For each application in Yii there is at least one bootstrap file. For web applications it's typically `index.php`, for
+console applications it's `yii`. Both are doing nearly the same job:
+
+1. Setting common constants.
+2. Including Yii itself.
+3. Including Composer autoloader.
+4. Reading config file into `$config`.
+5. Creating new application instance using `$config` and running it.
+
+Bootstrap file is not the part of framework but your application so it's OK to adjust it to fit your application. Typical
+adjustments are the value of `YII_DEBUG` that should never be `true` on production and the way config is read.
+
+Configuring application instance
+--------------------------------
+
+It was mentioned above that application is configured in bootstrap file when its instance is created. Config is typically
+stored in a PHP file in `/config` directory of the application and looks like the following:
+
+```php
+<?php
+return array(
+	'id' => 'applicationId',
+	'basePath' => dirname(__DIR__),
+	'components' => array(
+		// ...
+	),
+	'params' => require(__DIR__ . '/params.php'),
+);
+```
+
+In the above array keys are names of application properties. Depending on application type you can check properies of
+either `\yii\web\Application` or `\yii\console\Application`. Both are extended from `\yii\base\Application`.
+
+> Note that you can configure not only public class properties but anything accessible via setter. For example, to
+  configure runtime path you can use key named `runtimePath`. There's no such property in the application class but
+  since there's a corresponding setter named `setRuntimePath` it will be properly configured.
+
+Configuring application components
+----------------------------------
+
+Majority of Yii functionality are application components. These are attached to application via its `components` property:
+
+```php
+<?php
+return array(
+	'id' => 'applicationId',
+	'basePath' => dirname(__DIR__),
+	'components' => array(
+		'cache' => array(
+			'class' => 'yii\caching\FileCache',
+		),
+		'user' => array(
+			'identityClass' => 'app\models\User',
+		),
+		'errorHandler' => array(
+			'errorAction' => 'site/error',
+		),
+		'log' => array(
+			'traceLevel' => YII_DEBUG ? 3 : 0,
+			'targets' => array(
+				array(
+					'class' => 'yii\log\FileTarget',
+					'levels' => array('error', 'warning'),
+				),
+			),
+		),
+	),
+	// ...
+);
+```
+
+In the above four components are configured: `cache`, `user`, `errorHandler`, `log`. Each entry key is a component ID
+and the value is the configuration array. ID is used to access the component like `\Yii::$app->myComponent`.
+Configuration array has one special key named `class` that sets component class. The rest of the keys and values are used
+to configure component properties in the same way as top-level keys are used to configure application properties.
+
+Each application has predefined set of the components. In case of configuring one of these `class` key is omitted and
+application default class is used instead. You can check `registerCoreComponents` method of the application you are using
+to get a list of component IDs and corresponding classes.
+
+Note that Yii is smart enough to configure the component when it's actually used i.e. if `cache` is never used it will
+not be instantiated and configured at all.
+
+Setting component defaults classwide
+------------------------------------
+
+TBD
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