Update unit_testing.rst

Author: fzsys

create_framework/unit_testing.rst

@@ -7,6 +7,13 @@ must be sure that it behaves as advertised. If not, all the applications based
 on it will exhibit the same bugs. The good news is that whenever you fix a
 bug, you are fixing a bunch of applications too.
 
+At first, add a (development-time) dependency on phpunit/phpunit to your
+``composer.json`` file:
+
+.. code-block:: install
+
+    composer require --dev phpunit/phpunit
+
 Today's mission is to write unit tests for the framework we have created by
 using `PHPUnit`_. Create a PHPUnit configuration file in
 ``example.com/phpunit.xml.dist``: