Added a word of Adapter pattern and Liskov (#127)

Added a word on Adapter pattern and Liskov substitution principle

Author: Nyholm

clients.rst

@@ -1,12 +1,14 @@
 Clients & Adapters
 ==================
 
-There are clients implementing one of the HTTPlug interfaces directly,
-and adapter packages that implement the interface and forward the calls to HTTP clients not implementing the interface.
+There are two types of libraries you can use to send HTTP messages; clients and adapters. A client implements the
+``HttpClient`` and/or the ``HttpAsyncClient`` interfaces directly. A client adapter is a class implementing the
+interface and forwarding the calls to an HTTP client not implementing the interface. (See `Adapter pattern`_ on Wikipedia).
 
-.. _`php-http/client-implementation`: https://packagist.org/providers/php-http/client-implementation
-.. _`php-http/async-client-implementation`: https://packagist.org/providers/php-http/async-client-implementation:
+.. note::
 
+    All clients and adapters comply with `Liskov substitution principle`_ which means that you can easily change one
+    for another without any side effects.
 
 Clients:
 
@@ -29,9 +31,13 @@ Composer Virtual Packages
 -------------------------
 
 Virtual packages are a way to specify the dependency on an implementation of an interface-only repository
-without forcing a specific implementation. For HTTPlug, the virtual package is called ``php-http/client-implementation``.
+without forcing a specific implementation. For HTTPlug, the virtual packages are called `php-http/client-implementation`_
+and `php-http/async-client-implementation`_.
 
-There is no project registered with that name. However, all client implementations including client adapters for
+There is no library registered with those names. However, all client implementations (including client adapters) for
 HTTPlug use the ``provide`` section to tell composer that they do provide the client-implementation.
 
-
+.. _`php-http/client-implementation`: https://packagist.org/providers/php-http/client-implementation
+.. _`php-http/async-client-implementation`: https://packagist.org/providers/php-http/async-client-implementation
+.. _`Adapter pattern`: https://en.wikipedia.org/wiki/Adapter_pattern
+.. _`Liskov substitution principle`: https://en.wikipedia.org/wiki/Liskov_substitution_principle

spelling_word_list.txt

@@ -29,4 +29,5 @@ toolbar
 username
 whitelist
 wiki
+Wikipedia
 workflow