[HttpClient] Add Klaxoon as a backer to the README

Author: nicolas-grekas

README.md

@@ -3,6 +3,16 @@ HttpClient component
 
 The HttpClient component provides powerful methods to fetch HTTP resources synchronously or asynchronously.
 
+Sponsor
+-------
+
+The Httpclient component for Symfony 5.4/6.0 is [backed][1] by [Klaxoon][2].
+
+Klaxoon is a platform that empowers organizations to run effective and
+productive workshops easily in a hybrid environment. Anytime, Anywhere.
+
+Help Symfony by [sponsoring][3] its development!
+
 Resources
 ---------
 
@@ -11,3 +21,7 @@ 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://klaxoon.com
+[3]: https://symfony.com/sponsor