Add warning because of missing locking

I have no idea if locking can somehow be implemented in this session handler - this warning must do it for the time being.

Author: SvenRtbg

README.md

@@ -24,7 +24,9 @@ $config = ['ttl'=>3600, 'prefix'=>'foobar'];
 $sessionHandler = new Psr6SessionHandler($pool, $config);
 ```
 
+Note that this session handler does no kind of locking, so it will lose or overwrite your session data if you run scripts concurrently. You have been warned.
+
 ### Contribute
 
 Contributions are very welcome! Send a pull request to the [main repository](https://github.com/php-cache/cache) or 
-report any issues you find on the [issue tracker](http://issues.php-cache.com).
+report any issues you find on the [issue tracker](http://issues.php-cache.com).