Added instructions for installing dev testing package

doc/testing.rst

@@ -76,6 +76,16 @@ use it by making your test extend it::
     To make your application testable, you need to make sure you follow "Reusing
     applications" instructions from :doc:`usage`.
     
+.. note::
+
+    If you want to use the Symfony2 `WebTestCase` you will need to explicitly
+    install its dependencies for your project. Add the following to your
+    `composer.json` file.
+
+        "require-dev":{
+            "symfony/browser-kit": ">=2.3,<2.4-dev"
+        }
+
 For your WebTestCase, you will have to implement a ``createApplication``
 method, which returns your application. It will probably look like this::