Mention Platform.sh on README.

Author: GuGuss

README.markdown

@@ -14,3 +14,8 @@ Contributing
 We love contributors! For more information on how you can contribute to the
 Symfony documentation, please read
 [Contributing to the Documentation](https://symfony.com/doc/current/contributing/documentation/overview.html)
+
+Platform.sh
+-----------
+
+Pull requests are automatically built by [Platform.sh](https://platform.sh).