Update README.md

Author: felipelube

README.md

@@ -17,6 +17,7 @@ The following ENV vars are used to configure the app:
 ## Responses
 The possible responses are:
 - HTTP 204: task execution was successful, the output is logged to `stdout`.
+- HTTP 400: invalid request.
 - HTTP 403: invalid request or failed token authentication.
 - HTTP 405: only http `POST` method is allowed.
 - HTTP 500: task execution incurred in errors, traceback is logged to `stderr`.