[Security] Add SymfonyCasts as a backer to the READMEs

Author: nicolas-grekas

README.md

@@ -6,6 +6,17 @@ which makes it possible to easily separate the actual authorization logic from
 so called user providers that hold the users credentials. It is inspired by
 the Java Spring framework.
 
+Sponsor
+-------
+
+The Security component for Symfony 5.4/6.0 is [backed][1] by [SymfonyCasts][2].
+
+Learn Symfony faster by watching real projects being built and actively coding
+along with them. SymfonyCasts bridges that learning gap, bringing you video
+tutorials and coding challenges. Code on! 
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
@@ -14,3 +25,7 @@ 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://symfonycasts.com
+[3]: https://symfony.com/sponsor