Added note about sessions (#79)

* Added note about sessions

* Update README.md

Co-authored-by: Alexander Schranz <alexander@sulu.io>

Co-authored-by: Alexander Schranz <alexander@sulu.io>

Author: Nyholm

README.md

@@ -118,6 +118,13 @@ The runtime for [Symfony](https://symfony.com/) is included in the runtime compo
 
 * https://github.com/symfony/runtime
 
+## Note about sessions
+
+On Symfony < 5.4 session data will not be properly stored when using a "non-traditional"
+webserver like Bref, Google, ReactPHP, RoadRunner or Swoole. This problem (or missing
+feature) has been added in Symfony 5.4 and 6.0. You need to use the unreleased version
+of `symfony/http-kernel` for sessions to work properly.
+
 ## Contribute
 
 Contributions are always welcomed. Send your PR or open an issue here: https://github.com/php-runtime/runtime