[Notifier] Add Mercure.rocks as a backer to the README

Author: nicolas-grekas

README.md

@@ -3,6 +3,19 @@ Notifier Component
 
 The Notifier component sends notifications via one or more channels (email, SMS, ...).
 
+Sponsor
+-------
+
+The Notifier component for Symfony 5.4/6.0 is [backed][1] by [Mercure.rocks][2].
+
+Create real-time experiences in minutes! Mercure.rocks provides a realtime API service
+that is tightly integrated with Symfony: create UIs that update in live with UX Turbo,
+send notifications with the Notifier component, expose async APIs with API Platform and
+create low level stuffs with the Mercure component. We maintain and scale the complex
+infrastructure for you!
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
@@ -11,3 +24,7 @@ Resources
  * [Report issues](https://github.com/symfony/symfony/issues) and
    [send Pull Requests](https://github.com/symfony/symfony/pulls)
    in the [main Symfony repository](https://github.com/symfony/symfony)
+
+[1]: https://symfony.com/backers
+[2]: https://mercure.rocks
+[3]: https://symfony.com/sponsor