[client] Add docs for rpc client

Author: makasim

docs/client/rpc_call.md

@@ -0,0 +1,75 @@
+# Client. RPC call
+
+
+## The client side
+
+There is a handy class RpcClient shipped with the client component. 
+It allows you to easily send a message and wait for a reply.
+ 
+```php
+<?php
+use Enqueue\Client\SimpleClient;
+use Enqueue\Client\RpcClient;
+
+/** @var \Enqueue\Psr\PsrContext $context */
+
+
+$client = new SimpleClient($context);
+$rpcClient = new RpcClient($client->getProducer(), $context);
+
+$replyMessage = $rpcClient->call('greeting_topic', 'Hi Thomas!', 5);
+```
+
+You can perform several requests asynchronously with `callAsync` and request replays later.
+ 
+```php
+<?php
+use Enqueue\Client\SimpleClient;
+use Enqueue\Client\RpcClient;
+
+/** @var \Enqueue\Psr\PsrContext $context */
+
+
+$client = new SimpleClient($context);
+$rpcClient = new RpcClient($client->getProducer(), $context);
+
+$promises = [];
+$promises[] = $rpcClient->callAsync('greeting_topic', 'Hi Thomas!', 5);
+$promises[] = $rpcClient->callAsync('greeting_topic', 'Hi Thomas!', 5);
+$promises[] = $rpcClient->callAsync('greeting_topic', 'Hi Thomas!', 5);
+$promises[] = $rpcClient->callAsync('greeting_topic', 'Hi Thomas!', 5);
+
+$replyMessages = [];
+foreach ($promises as $promise) {
+    $replyMessages[] = $promise->getMessage();
+}
+```
+
+## The server side
+
+On the server side you may register a processor which returns a result object with a reply message set.
+Of course it is possible to implement rpc server side based on transport classes only. That would require a bit more work to do. 
+
+```php
+<?php
+
+use Enqueue\Client\SimpleClient;
+use Enqueue\Psr\PsrMessage;
+use Enqueue\Psr\PsrContext;
+use Enqueue\Consumption\Result;
+use Enqueue\Consumption\ChainExtension;
+use Enqueue\Consumption\Extension\ReplyExtension;
+
+/** @var \Enqueue\Psr\PsrContext $context */
+
+$client = new SimpleClient($this->context);
+$client->bind('greeting_topic', 'greeting_processor', function (PsrMessage $message, PsrContext $context) use (&$requestMessage) {
+    echo $message->getBody();
+    
+    return Result::reply($context->createMessage('Hi there! I am John.'));
+});
+
+$client->consume(new ChainExtension([new ReplyExtension()]));
+```
+
+[back to index](../index.md)

docs/index.md

@@ -13,6 +13,7 @@
     - [Message examples](client/message_examples.md)
     - [Supported brokers](client/supported_brokers.md)
     - [Message bus](client/message_bus.md)
+    - [RPC call](client/rpc_call.md)
 * Job queue
     - [Run unique job](job_queue/run_unique_job.md)
     - [Run sub job(s)](job_queue/run_sub_job.md)