add comment to README about the Middleware as a replacement for the e…

…ID parameter in your payment extensions

README.md

@@ -1,2 +1,20 @@
 The extension transactor has the purpose to enable online payments connecting to various gateways.
 
+You must configure it in the Extension Configuration of the Settings backend module.
+Set the compatibility mode (basic.compatibility) to 0 for the new API.
+Older extension versions might need the old API and a 1 here.
+
+
+Since version 0.9.0 the transactor extension contains a middleware, which payment extensions can use for a gateway instant message after the payment.
+
+Use the transactor parameter as
+transactor=mygateway
+in the backend url to your payment extension page.
+
+Your extension`s ext_localconf.php file must configure the middleware in TYPO3 10+:
+
+    $GLOBALS['TYPO3_CONF_VARS']['FE']['transactor_include']['mygateway'] =  \Foo\Bar\Controller\ServerResponseController::class . '::processRequest';
+
+Then your class ServerResponseController can receive the calls from the instant messages from your gateway.
+
+