Add documentation for the MockClientStrategy (#173)

* Add documentation for the MockClientStrategy

See [#90](php-http/discovery#90).

* Update discovery.rst

* Break lines at 80 characters.

Author: Sam Rapaport

discovery.rst

@@ -13,6 +13,7 @@ Currently available discovery services:
 - PSR-7 Message Factory Discovery
 - PSR-7 URI Factory Discovery
 - PSR-7 Stream Factory Discovery
+- Mock Client Discovery (not enabled by default)
 
 The principle is always the same: you call the static ``find`` method on the discovery service if no explicit
 implementation was specified. The discovery service will try to locate a suitable implementation.
@@ -233,6 +234,54 @@ This type of discovery finds a URI factory for a PSR-7_ URI implementation::
         }
     }
 
+Mock Client Discovery
+---------------------------
+
+You may find yourself testing parts of your application that are dependent on an
+HTTP Client using the Discovery Service, but do not necessarily need to perform
+the request nor contain any special configuration. In this case, the
+``Http\Mock\Client`` from the ``php-http/mock-client`` package is typically used
+to fake requests and keep your tests nicely decoupled. However, for the best
+stability in a production environment, the mock client is not set to be found
+via the Discovery Service. Attempting to run a test which relies on discovery
+and uses a mock client will result in an ``Http\Discovery\Exception\NotFoundException``.
+Thankfully, Discovery gives us a Mock Client strategy that can be added straight
+to the Discovery. Let's take a look::
+
+    use MyCustomService;
+    use Http\Mock\Client as MockClient;
+    use Http\Discovery\HttpClientDiscovery;
+    use Http\Discovery\Strategy\MockClientStrategy;
+
+    class MyCustomServiceTest extends TestCase
+    {
+        public function setUp()
+        {
+            HttpClientDiscovery::prependStrategy(MockClientStrategy::class);
+
+            $this->service = new MyCustomService;
+        }
+
+        public function testMyCustomServiceDoesSomething()
+        {
+            // Test...
+        }
+    }
+    
+In the example of a test class above, we have our ``MyCustomService`` which
+relies on an HTTP Client implementation. We do not need to test that the actual
+request our custom service makes is successful in this test class, so it makes
+sense to use the Mock Client. However, we do want to make sure that our
+dependency injection using the Discovery service properly works, as this is a
+major feature of our service. By calling the ``HttpClientDiscovery``'s
+``prependStrategy`` method and passing in the ``MockClientStrategy`` namespace,
+we have now added the ability to discover the mock client and our tests will
+work as desired.
+
+It is important to note that you must explicitly enable the ``MockClientStrategy``
+and that it is not used by the Discovery Service by default. It is simply
+provided as a convenient option when writing tests.
+
 Integrating your own implementation with the discovery mechanism using Puli
 ---------------------------------------------------------------------------