minor #37542 [Mailer] Updated README for the Mailer component (wouterj)

This PR was merged into the 4.4 branch.

Discussion
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[Mailer] Updated README for the Mailer component

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#13945

I think it's worth adding a second example showcasing the twig integration for 2 reasons:

* If you're used to use this component in the framework, I think it's likely you want Twig integration in standalone apps.
* The integration actually lives in `symfony/twig-bridge`, it won't be very easy to catch while reading the component code - unless you're very comfortable with the Symfony architecture.

Commits
-------

696560c690 Updated README for the Mailer component

Author: fabpot

README.md

@@ -3,9 +3,66 @@ Mailer Component
 
 The Mailer component helps sending emails.
 
+Getting Started
+---------------
+
+```
+$ composer require symfony/mailer
+```
+
+```php
+use Symfony\Component\Mailer\Transport;
+use Symfony\Component\Mailer\Mailer;
+
+$transport = Transport::fromDsn('sendgrid://KEY@default');
+$mailer = new Mailer($transport);
+
+$email = (new Email())
+    ->from('[email protected]')
+    ->to('[email protected]')
+    //->cc('[email protected]')
+    //->bcc('[email protected]')
+    //->replyTo('[email protected]')
+    //->priority(Email::PRIORITY_HIGH)
+    ->subject('Time for Symfony Mailer!')
+    ->text('Sending emails is fun again!')
+    ->html('<p>See Twig integration for better HTML integration!</p>');
+
+$mailer->send($email);
+```
+
+To enable the Twig integration of the Mailer, require `symfony/twig-bridge` and
+set up the `BodyRenderer`:
+
+```php
+use Symfony\Bridge\Twig\Mime\BodyRenderer;
+use Symfony\Component\EventDispatcher\EventDispatcher;
+use Symfony\Component\Mailer\EventListener\MessageListener;
+use Twig\Environment as TwigEnvironment;
+
+$twig = new TwigEnvironment(...);
+$messageListener = new MessageListener(new BodyRenderer($twig));
+
+$eventDispatcher = new EventDispatcher();
+$eventDispatcher->addSubscriber($messageListener);
+
+$mailer = new Mailer($transport, null, $eventDispatcher);
+
+$email = (new TemplatedEmail())
+    // ...
+    ->htmlTemplate('emails/signup.html.twig')
+    ->context([
+        'expiration_date' => new \DateTime('+7 days'),
+        'username' => 'foo',
+    ])
+;
+$mailer->mail($email);
+```
+
 Resources
 ---------
 
+  * [Documentation](https://symfony.com/doc/current/mailer.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)