README: add information about use with setup-php

As of version 2.15.0 of the `shivammathur/setup-php` action for GitHub Actions, the PHPUnit Polyfills can be installed directly from this action using the `tools` key.

In that case, both this package as well as PHPUnit will be globally installed via Composer.

This commit adds an FAQ section to the readme with a little more information about how this works and how to get things running when installed like that.

README.md

@@ -690,6 +690,38 @@ Yes, this package can also be used when running tests via a PHPUnit Phar file.
 In that case, make sure that the `phpunitpolyfills-autoload.php` file is explicitly `require`d in the test bootstrap file.
 (Not necessary when the Composer `vendor/autoload.php` file is used as, or `require`d in, the test bootstrap.)
 
+
+### Q: How do I run my tests when the library is installed via the GitHub Actions `setup-php` action ?
+
+As of [shivammathur/setup-php](https://github.com/shivammathur/setup-php) version [2.15.0](https://github.com/shivammathur/setup-php/releases/tag/2.15.0), the PHPUnit Polyfills are available as one of the tools which can be installed directly by the Setup-PHP GitHub action.
+
+```yaml
+- name: Setup PHP with tools
+  uses: shivammathur/setup-php@v2
+  with:
+    php-version: '8.0'
+    tools: phpunit-polyfills
+```
+
+The above step will install both the PHPUnit Polyfills, as well as PHPUnit, as Composer global packages.
+
+After this step has run, you can run PHPUnit, like you would normally, by using `phpunit`.
+```yaml
+- name: Run tests
+  run: phpunit
+```
+
+:point_right: If you rely on Composer for autoloading your project files, you will still need to run `composer autoload --dev` and include the project local `vendor/autoload.php` file as/in your test bootstrap.
+
+> :mortar_board: Why this works:
+>
+> Composer will place all files in the global Composer `bin` directory in the system path and the Composer installed PHPUnit version will load the Composer global `autoload.php` file, which will automatically also load the PHPUnit Polyfills.
+
+Now you may wonder, _"what about if I explicitly request both `phpunit` as well as `phpunit-polyfills` in `tools`?"_
+
+In that case, when you run `phpunit`, the PHPUnit PHAR will not know how to locate the PHPUnit Polyfills, so you will need to do some wizardry in your test bootstrap to get things working.
+
+
 ### Q: How can I verify the version used of the PHPUnit Polyfills library ?
 
 For complex test setups, like when the Polyfills are provided via a test suite dependency, or may already be loaded via an overarching project, it can be useful to be able to check that a version of the package is used which complies with the requirements for your test suite.