Added documentation

Author: GeeH

doc/book/config-abstract-factory.md

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+# Config Abstract Factory
+
+You can simplify the process of creating factories by adding the 
+`ConfigAbstractFactory` to your service manager. This allows you to define
+services using a configuration map, rather than having to create separate 
+factories for all your services. 
+
+## Enabling
+You can enable the `ConfigAbstractFactory` in the same way that you would enable 
+any other abstract factory - in your own code:
+
+```php
+$serviceManager = new ServiceManager();
+$serviceManager->addAbstractFactory(new ConfigAbstractFactory());
+```
+
+Or within any config provider using:
+
+```php
+return [
+    'service_manager' => [
+        'abstract_factories' => [
+            ConfigAbstractFactories::class,
+        ],
+    ],
+];
+```
+
+## Configuring
+
+Configuration is done through the `config` service manager key, in an array with
+the key `Zend\ServiceManager\AbstractFactory\ConfigAbstractFactory`. If you are using 
+config merging from the MVC/ModuleManager, in this just means that you can 
+add a `ConfigAbstractFactory::class` key to your merged config which contains service
+definitions:
+
+```php
+use Zend\ServiceManager\AbstractFactory\ConfigAbstractFactory;
+
+return [
+    ConfigAbstractFactory::class => [
+        MyInvokableClass::class => [],
+        MySimpleClass::class => [
+            Logger::class,          
+        ],
+        Logger::class => [
+            Handler::class,
+        ],
+    ],
+];
+```
+
+The definition tells the service manager how this abstract factory should manage dependencies in
+the classes defined. In the above example, `MySimpleClass` has a single dependency on a `Logger`
+instance. The abstract factory will simply look to fulfil that dependency by calling a `get` 
+call with that key on the service manager it is attached to. In this way, you can create the 
+correct tree of dependencies to successfully return any given service. Note that `Handler` does not have a 
+configuration for the abstract factory, but this would work if `Handler` had a traditional factory and 
+can be created by this service manager.