add phpdocs to Executor class

add InvalidReturnTypeError class

Author: joonlabs

src/Errors/InvalidReturnTypeError.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace GraphQL\Errors;
+
+class InvalidReturnTypeError extends GraphQLError
+{
+    protected $code = "INVALID_RETURN_TYPE";
+}
+
+?>