Update Readme with new option.

Author: DotNetSimon

README.md

@@ -349,6 +349,19 @@ WebhookCall::create()
 ```
 or activate the `throw_exception_on_failure` global option of the `webhook-server` config file.
 
+### Sending raw string body instead of JSON
+
+By default, all webhooks will transform the payload into JSON. Instead of sending JSON, you can send any string by using the `sendRawBody(string $body)` option instead.
+
+Due to type mismatch in the Signer API, it is currently not support to sign raw data requests
+```php
+WebhookCall::create()
+    ->sendRawBody("<root>someXMLContent</root>")
+    ->doNotSign()
+    ...
+    ->dispatch();
+```
+
 ### Events
 
 The package fires these events: