Add call to backers in README files

Author: nicolas-grekas

README.md

@@ -44,6 +44,8 @@ $url = $generator->generate('blog_show', [
 Sponsor
 -------
 
+This package is looking for a [backer][1].
+
 Help Symfony by [sponsoring][3] its development!
 
 Resources
@@ -55,4 +57,5 @@ Resources
    [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