update readme

Author: dbu

README.md

@@ -21,12 +21,15 @@ $ composer require php-http/mock-client
 
 ## Usage
 
-Use this client in a testing framework.
+This client does not actually send requests to any server.
+
+Instead it stores the request and returns a pre-set response or throws an exception. This client is useful for unit
+testing code that depends on a HTTPlug client to send requests and receive responses.
 
 
 ## Documentation
 
-Please see the [official documentation](http://docs.php-http.org).
+Please see the [mock client](http://docs.php-http.org/en/latest/clients/mock-client.html) section in the [official documentation](http://docs.php-http.org).
 
 
 ## Testing
@@ -48,7 +51,7 @@ If you discover any security related issues, please contact us at [security@php-
 
 ## Credits
 
-Thanks to [David de Boer](https://github.com/ddeboer) for implementing this client.
+Thanks to [David de Boer](https://github.com/ddeboer) for implementing the mock client.
 
 
 ## License