[docs] add docs.

Author: makasim

docs/bundle/consumption_extension.md

@@ -0,0 +1,41 @@
+# Consumption extension
+
+Here, I show how you can crate a custom extension and register it.
+Let's first create an extension itself:
+
+```php
+<?php
+// src/AppBundle/Enqueue;
+namespace AppBundle\Enqueue;
+
+use Enqueue\Consumption\ExtensionInterface;
+use Enqueue\Consumption\EmptyExtensionTrait;
+use Enqueue\Consumption\Context;
+
+class CountProcessedMessagesExtension implements ExtensionInterface
+{
+    use EmptyExtensionTrait;
+    
+    private $processedMessages = 0;
+    
+    /**
+     * {@inheritdoc}  
+     */
+    public function onPostReceived(Context $context)
+    {
+        $this->processedMessages += 1;
+    }
+}
+```
+
+Now we have to register as a Symfony service with special tag:
+
+```yaml
+services:
+    app.enqueue.count_processed_messages_extension:
+        class: 'AppBundle\Enqueue\CountProcessedMessagesExtension'
+        tags:
+            - { name: 'enqueue.consumption.extension', priority: 10 }
+```
+
+[back to index](../index.md)

docs/consumption/extensions.md

@@ -1,6 +1,7 @@
 # Consumption extensions.
 
-
+You can learn how to register extensions in [quick tour](../quick_tour.md#consumption).
+There's dedicated [chapter](../bundle/consumption_extension.md) for how to add extension in Symfony app.
 
 ## [LoggerExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/LoggerExtension.php) 
 

docs/index.md

@@ -12,6 +12,7 @@
     - [Config reference](bundle/config_reference.md)
     - [Cli commands](bundle/cli_commands.md)
     - [Job queue](bundle/job_queue.md)
+    - [Consumption extension](bundle/consumption_extension.md)
     - [Production settings](bundle/production_settings.md)
     - [Debuging](bundle/debuging.md)
     - [Functional testing](bundle/functional_testing.md)