minor #14913 [Testing] [PHPUnitBridge] Docs for the new SYMFONY_PHPUNIT_REQUIRE env variable (acasademont)

This PR was merged into the 5.3-dev branch.

Discussion
----------

[Testing] [PHPUnitBridge] Docs for the new SYMFONY_PHPUNIT_REQUIRE env variable

Code PR: symfony/symfony#40059

Commits
-------

79a2112 Docs for the new SYMFONY_PHPUNIT_REQUIRE env variable

Author: javiereguiluz

components/phpunit_bridge.rst

@@ -930,6 +930,18 @@ If you have installed the bridge through Composer, you can run it by calling e.g
     then set the ``SYMFONY_PHPUNIT_REMOVE`` env var to ``symfony/yaml``.
 
     It's also possible to set this env var in the ``phpunit.xml.dist`` file.
+    
+.. tip::
+
+    It is also possible to require additional packages that will be installed along
+    the rest of the needed PHPUnit packages using the ``SYMFONY_PHPUNIT_REQUIRE``
+    env variable. This is specially useful for installing PHPUnit plugins without
+    having to add them to your main ``composer.json`` file.
+
+.. versionadded:: 5.3
+
+    The ``SYMFONY_PHPUNIT_REQUIRE`` env variable was introduced in
+    Symfony 5.3.
 
 Code Coverage Listener
 ----------------------