Add call to backers in README files

Author: nicolas-grekas

README.md

@@ -30,6 +30,13 @@ $passwordHasher->verify($hash, 'wrong'); // returns false
 $passwordHasher->verify($hash, 'plain'); // returns true (valid)
 ```
 
+Sponsor
+-------
+
+This package is looking for a [backer][1].
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
@@ -38,3 +45,6 @@ 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
+[3]: https://symfony.com/sponsor