Add call to backers in README files

Author: nicolas-grekas

README.md

@@ -7,10 +7,20 @@ information about the execution of any request.
 **Never** enable it on production servers as it will lead to major security
 vulnerabilities in your project.
 
+Sponsor
+-------
+
+This package is looking for a [backer][1].
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
  * [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)
    in the [main Symfony repository](https://github.com/symfony/symfony)
+
+[1]: https://symfony.com/backers
+[3]: https://symfony.com/sponsor