minor #38409 [Stopwatch] Added example to the README (wouterj)

This PR was merged into the 4.4 branch.

Discussion
----------

[Stopwatch] Added example to the README

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/14333

Commits
-------

e2461c90 Added Stopwatch example to the README

src/Symfony/Component/Stopwatch/README.md

@@ -3,10 +3,39 @@ Stopwatch Component
 
 The Stopwatch component provides a way to profile code.
 
+Getting Started
+---------------
+
+```
+$ composer require symfony/stopwatch
+```
+
+```php
+use Symfony\Component\Stopwatch\Stopwatch;
+
+$stopwatch = new Stopwatch();
+
+// optionally group events into sections (e.g. phases of the execution)
+$stopwatch->openSection();
+
+// starts event named 'eventName'
+$stopwatch->start('eventName');
+
+// ... run your code here
+
+// optionally, start a new "lap" time
+$stopwatch->lap('foo');
+
+// ... run your code here
+
+$event = $stopwatch->stop('eventName');
+
+$stopwatch->stopSection('phase_1');
+```
+
 Resources
 ---------
 
-  * [Documentation](https://symfony.com/doc/current/components/stopwatch.html)
   * [Contributing](https://symfony.com/doc/current/contributing/index.html)
   * [Report issues](https://github.com/symfony/symfony/issues) and
     [send Pull Requests](https://github.com/symfony/symfony/pulls)