Add info about addMetadata method to README

Author: iv-craig

README.md

@@ -110,6 +110,7 @@ public function routeNotificationForInterfax($notification)
 `file(string $file)` : Accepts the full path to a single file (full list of supported file types [found here](https://www.interfax.net/en/help/supported_file_types)).  
 `files(array $array)` : Accepts an array of file paths.  
 `stream(Filestream $stream, string $name)` : Accepts a file stream.
+`addMetadata(array $data)`: Add metadata for logging purposes in case of an error.
 
 ## Changelog