Add call to backers in README files

Author: nicolas-grekas

README.md

@@ -7,7 +7,9 @@ machine.
 Sponsor
 -------
 
-Help Symfony by [sponsoring][1] its development!
+This package is looking for a [backer][1].
+
+Help Symfony by [sponsoring][3] its development!
 
 Resources
 ---------
@@ -18,4 +20,5 @@ Resources
    [send Pull Requests](https://github.com/symfony/symfony/pulls)
    in the [main Symfony repository](https://github.com/symfony/symfony)
 
-[1]: https://symfony.com/sponsor
+[1]: https://symfony.com/backers
+[3]: https://symfony.com/sponsor