[doc][skip ci] add doc for client on send extensions.

docs/client/extensions.md

@@ -0,0 +1,45 @@
+# Client extensions.
+
+There is an ability to hook into sending process. You have to create an extension class that implements `Enqueue\Client\ExtensionInterface` interface.
+For example, `TimestampMessageExtension` extension adds timestamps every message before sending it to MQ. 
+
+```php
+<?php
+namespace Acme;
+
+use Enqueue\Client\ExtensionInterface;
+use Enqueue\Client\Message;
+
+class TimestampMessageExtension implements ExtensionInterface
+{
+    public function onPreSend($topic, Message $message)
+    {
+        if ($message->getTimestamp()) {
+            $message->setTimestamp(time());
+        }
+    }
+
+    public function onPostSend($topic, Message $message)
+    {
+
+    }
+} 
+```
+
+## Symfony
+
+To use the extension in Symfony, you have to register it as a container service with a special tag. 
+
+```yaml
+# config/services.yaml
+
+services:
+  timestamp_message_extension:
+    class: Acme\TimestampMessageExtension
+    tags:
+      - { name: 'enqueue.client.extensions' }    
+```
+
+You can add `priority` attribute with a number. The higher value you set the earlier the extension is called.  
+
+[back to index](../index.md)

docs/index.md

@@ -22,6 +22,7 @@
     - [Supported brokers](client/supported_brokers.md)
     - [Message bus](client/message_bus.md)
     - [RPC call](client/rpc_call.md)
+    - [Extensions](client/extensions.md)
 * [Job queue](#job-queue)
     - [Run unique job](job_queue/run_unique_job.md)
     - [Run sub job(s)](job_queue/run_sub_job.md)