Improving the usage section

* added examples on how to use the PSR-7 standard:
  - using it to create your own implementation
  - consuming it with an real world api service example

Author: ibrahimab

README.md

@@ -10,4 +10,115 @@ interface that describes a HTTP message. See the specification for more details.
 Usage
 -----
 
-We'll certainly need some stuff in here.
+There are two ways of using the [PSR-7](http://www.php-fig.org/psr/psr-7) standard. You can write your own implementation or consume a library/package that implements the standard.
+
+* [Writing your own implementation](https://packagist.org/providers/psr/http-message-implementation)
+* [Consuming the interfaces through a vendor](#consuming-through-vendor)
+
+<a name="consuming-through-vendor"></a>
+## Consuming the interfaces through a vendor
+Imagine you need a way to send HTTP requests from your application. Instead of reinventing the wheel, you would then pull in a vendor that has done the legwork for you (perhaps through composer) and use that.
+
+As an example, the [Guzzle](https://github.com/guzzle/guzzle) library already supports [PSR-7](http://www.php-fig.org/psr/psr-7). It is then possible to send a request and get back a response.
+To decouple your code from guzzle, you can create your own client interface
+and have that type hinted in your services. Your services know to send a RequestInterface to the client and that they get back a ResponseInterface object.
+
+You have an application with an api that allows you to fetch posts. You create a RequestInterface implementation object (like Guzzle's Request object) and send it to the client. The client then responds with an implementation of a ResponseInterface object (like Guzzle's Response object).
+
+```php
+<?php
+namespace YourApp\Controller;
+
+use Psr7\Http\Message\RequestInterface;
+use Psr7\Http\Message\ResponseInterface;
+use GuzzleHttp\Psr7\Client;
+use GuzzleHttp\Psr7\Request;
+
+/**
+ * Creating a client interface that your services can depend on so you can 
+ * swap out clients whenever you feel like it, without changing the underlying 
+ * code
+ */
+interface ClientInterface
+{
+    /**
+     * This method sends a request and returns a response
+     * 
+     * @param  RequestInterface $request
+     * 
+     * @return ResponseInterface
+     */
+    public function send(RequestInterface $request);
+
+    /**
+     * @param string $method
+     * @param string $url
+     * 
+     * @return RequestInterface
+     */
+    public function request($method, $url);
+}
+
+/**
+ * This is a guzzle client, implementing our interface
+ */
+class GuzzleClient implements ClientInterface
+{
+    public function send(RequestInterface $request)
+    {
+        return $this->guzzle->send($request);
+    }
+
+    public function request($method, $url)
+    {
+        return new Request($method, $url);
+    }
+}
+
+/**
+ * Or another client, that you swapped guzzle out with
+ * as a simple example.
+ */
+class OtherHttpClient implements ClientInterface
+{
+    public function send(RequestInterface $request)
+    {
+        $this->otherClient->send($request);
+    }
+
+    public function request($method, $url)
+    {
+        return new OtherHttpVendorRequest($method, $url);
+    }
+}
+
+/**
+ * Your posts service then receives a client object, on which he can create 
+ * requests (RequestInterface) and receive responses (ResponseInterface)
+ */
+class PostsService
+{
+    /**
+     * Inject the client, but keep the service agnostic as to which client it is using.
+     * 
+     * @param ClientInterface $client
+     */
+    public function __construct(ClientInterface $client)
+    {
+        $this->client = $client;
+    }
+
+    /**
+     * @return array
+     */
+    public function all()
+    {
+        // $request is now an instance of RequestInterface
+        $request = $this->client->request('GET', 'https://api.app.com/posts');
+
+        // $response is now an instance of ResponseInterface
+        $response = $this->client->send($request);
+
+        return json_decode($response->getBody(), true);
+    }
+}