minor #16269 [Process] tweaked README (fabpot)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #16269).

Discussion
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[Process] tweaked README

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #16216
| License       | MIT
| Doc PR        | n/a

Commits
-------

11622ad [Process] tweaked README

src/Symfony/Component/Process/README.md

@@ -7,12 +7,13 @@ In this example, we run a simple directory listing and get the result back:
 
 ```php
 use Symfony\Component\Process\Process;
+use Symfony\Component\Process\Exception\ProcessFailedException;
 
 $process = new Process('ls -lsa');
 $process->setTimeout(3600);
 $process->run();
 if (!$process->isSuccessful()) {
-    throw new RuntimeException($process->getErrorOutput());
+    throw new ProcessFailedException($process);
 }
 
 print $process->getOutput();
@@ -21,6 +22,19 @@ print $process->getOutput();
 You can think that this is easy to achieve with plain PHP but it's not especially
 if you want to take care of the subtle differences between the different platforms.
 
+You can simplify the code by using `mustRun()` instead of `run()`, which will
+throw a `ProcessFailedException` automatically in case of a problem:
+
+```php
+use Symfony\Component\Process\Process;
+
+$process = new Process('ls -lsa');
+$process->setTimeout(3600);
+$process->mustRun();
+
+print $process->getOutput();
+```
+
 And if you want to be able to get some feedback in real-time, just pass an
 anonymous function to the ``run()`` method and you will get the output buffer
 as it becomes available: