Forgot doc for 401a399

Author: Léo Bonnargent

README.md

@@ -108,6 +108,7 @@ DEBUG=preview-email node app.js
   * `dir` (String) - a path to a directory for saving the generated email previews (defaults to `os.tmpdir()`, see [os docs](https://nodejs.org/api/os.html#os_os_tmpdir) for more insight)
   * `open` (Object or Boolean) - an options object that is passed to [open][] (defaults to `{ wait: false }`) - if set to `false` then it will not open the email automaitcally in the browser using [open][], and if set to `true` then it will default to `{ wait: false }`
   * `template` (String) - a file path to a `pug` template file (defaults to preview-email's [template.pug](template.pug) by default) - **this is where you can pass a custom template for rendering email previews, e.g. your own stylesheet**
+  * `urlTransform` (Function (path) => url) - a function to build preview url from file path (defaults to `(path) => 'file://[file path]'`) - _this is where you can customize the opened path to handle WSL to Windows tranformation or build a http url if `dir` is served._
 
 
 ## Contributors