Adds description for view helper layout

Author: froschdesign

doc/book/helpers/layout.md

@@ -0,0 +1,60 @@
+# Layout
+
+The `Layout` helper is used to get and set the template for the layout or to 
+retrieving the root view model.
+
+## Basic Usage
+
+### Change the Layout Template
+
+If you're running a zend-mvc application then the layout template is set in the
+configuration for the [`ViewManager`](https://docs.zendframework.com/zend-mvc/services/#viewmanager).
+
+To change the layout template within a view script, call:
+
+```php
+$this->layout('layout/backend');
+```
+
+Or use the `setTemplate` method:
+
+```php
+$this->layout()->setTemplate('layout/backend');
+```
+
+### Set View Variable on Layout Model
+
+The `Layout` helper can also retrieve the view model for the layout (root): 
+
+```php
+/** @var \Zend\View\Model\ViewModel $rootViewModel */
+$rootViewModel = $this->layout();
+```
+
+This offers the possibility to set variables for the layout script.
+
+#### Set a Single Variable
+
+```php
+$this->layout()->setVariable('infoText', 'Some text for later');
+```
+
+Use in your layout script:
+
+```php
+if (isset($infoText)) {
+    echo $infoText;
+}
+```
+
+#### Set a Set of Variables
+
+```php
+$this->layout()->setVariables([
+    'headerText' => '…',
+    'footerText' => '…',
+]);
+```
+
+More informations related to view models can be found in the
+[quick start](https://docs.zendframework.com/zend-view/quick-start/).