Add clarification on Promise usage in transform

README.md

@@ -126,9 +126,17 @@ Type: `Function`
 Default: `null`
 
 A function to transform the content of imported files. Take one argument (file
-  content) and should return the modified content or promise with it.
+  content) and should return the modified content or a resolved promise with it.
 `undefined` result will be skipped.
 
+```js
+transform: function(css) {
+  return postcss([somePlugin]).process(css).then(function(result) {
+    return result.css;
+  });
+}
+```
+
 #### `plugins`
 
 Type: `Array`