docs: clarify type of exported function (#48)

Author: MiniGod

README.md

@@ -73,8 +73,8 @@ And run `webpack` via your preferred method.
 
 ## Return Object Properties
 
-Targeted modules of this loader must export either a `Function` or `Promise`
-that returns an object containing a `code` property at a minimum, but can
+Targeted modules of this loader must export a `Function` that returns an object,
+or a `Promise` resolving an object (e.g. async function), containing a `code` property at a minimum, but can
 contain any number of additional properties.
 
 ### `code`