minor #16427 Update unit_testing.rst (fzsys)

This PR was submitted for the 6.0 branch but it was merged into the 4.4 branch instead.

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Update unit_testing.rst

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a76494f Update unit_testing.rst

Author: wouterj

create_framework/unit_testing.rst

@@ -8,8 +8,13 @@ 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.
 
 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``:
+using `PHPUnit`_. At first, install PHPUnit as a development dependency:
+
+.. code-block:: terminal
+
+    $ composer require --dev phpunit/phpunit
+
+Then, create a PHPUnit configuration file in ``example.com/phpunit.xml.dist``:
 
 .. code-block:: xml