started authorization docs

docs/guide/authorization.md

+Authorization
+=============
+
+Authorization is the process of verifying that user has enough permissions to do something. Yii provides several methods
+of controlling it.
+
+Access control basics
+---------------------
+
+Basic acces control is very simple to implement using [[\yii\web\AccessControl]]:
+
+```php
+class SiteController extends Controller
+{
+	public function behaviors()
+	{
+		return [
+			'access' => [
+				'class' => \yii\web\AccessControl::className(),
+				'only' => ['login', 'logout', 'signup'],
+				'rules' => [
+					[
+						'actions' => ['login', 'signup'],
+						'allow' => true,
+						'roles' => ['?'],
+					],
+					[
+						'actions' => ['logout'],
+						'allow' => true,
+						'roles' => ['@'],
+					],
+				],
+			],
+		];
+	}
+	// ...
+```
+
+In the code above we're attaching access control behavior to a controller. Since there's `only` option specified, it
+will be applied to 'login', 'logout' and 'signup' actions only. A set of rules that are basically options for
+[[\yii\web\AccessRule]] reads as follows:
+
+- Allow all guest (not yet authenticated) users to access 'login' and 'signup' actions.
+- Allow authenticated users to access 'logout' action.
+
+Rules are checked one by one from top to bottom. If rule matches, action takes place immediately. If not, next rule is
+checked. If no rules matched access is denied.
+
+[[\yii\web\AccessRule]] is quite flexible and allows additionally to what was demonstrated checking IPs and request method
+(i.e. POST, GET). If it's not enough you can specify your own check via anonymous function:
+
+```php
+class SiteController extends Controller
+{
+	public function behaviors()
+	{
+		return [
+			'access' => [
+				'class' => \yii\web\AccessControl::className(),
+				'only' => ['special'],
+				'rules' => [
+					[
+						'actions' => ['special'],
+						'allow' => true,
+						'matchCallback' => function ($rule, $action) {
+							return date('d-m') === '31-10';
+						}
+					],
+```
+
+Sometimes you want a custom action to be taken when access is denied. In this case you can specify `denyCallback`.
+
+Role based access control (RBAC)
+--------------------------------
+
+Role based access control is very flexible approach to controlling access that is a perfect match for complex systems
+where permissions are customizable.
+
+In order to start using it some extra steps are required. First of all we need to configure `authManager` application
+component:
+
+```php
+
+```
+
+Then create permissions hierarchy.
+
+Specify roles from RBAC in controller's access control configuration or call [[User::checkAccess()]] where appropriate.
+
+### How it works
+
+TBD: write about how it works with pictures :)
+
+### Avoiding too much RBAC
+
+In order to keep auth hierarchy simple and efficient you should avoid creating and using too much nodes. Most of the time
+simple checks could be used instead. For example such code that uses RBAC:
+
+```php
+public function editArticle($id)
+{
+  $article = Article::find($id);
+  if (!$article) {
+    throw new HttpException(404);
+  }
+  if (!\Yii::$app->user->checkAccess('edit_article', ['article' => $article])) {
+    throw new HttpException(403);
+  }
+  // ...
+}
+```
+
+can be replaced with simpler code that doesn't use RBAC:
+
+```php
+public function editArticle($id)
+{
+    $article = Article::find(['id' => $id, 'author_id' => \Yii::$app->user->id]);
+    if (!$article) {
+      throw new HttpException(404);
+    }
+    // ...
+}
+```