[Tests] added a note about installing PHPUnit dependencies

Author: HeahDude

testing.rst

@@ -31,6 +31,17 @@ command:
 
     $ ./vendor/bin/simple-phpunit
 
+.. note::
+
+    After installing the bridge, you should run once:
+
+    .. code-block:: terminal
+
+        $ ./vendor/bin/simple-phpunit install
+
+    That will install PHPUnit dependencies and make the classes available in
+    your project.
+
 PHPUnit is configured by the ``phpunit.xml.dist`` file in the root of your
 Symfony application.