chore: update README.md (#28)

Author: keradus

README.md

@@ -1,8 +1,3 @@
 # PHP-CS-Fixer/phpunit-constraint-xmlmatchesxsd
 
-This version is for [PHP CS Fixer](https://github.com/FriendsOfPHP/PHP-CS-Fixer) only! Do not use it!
-
-We hope to see this assert to be provided by PHPUnit itself,
-please help out if you can to make this happen.
-
-For questions visit us @ https://gitter.im/PHP-CS-Fixer/Lobby
+This constraint is for [PHP CS Fixer](https://github.com/FriendsOfPHP/PHP-CS-Fixer) usage only.