Update 'Whitespace' section

Add mention of editorconfig files to help ensure whitespace is consistent

source/manuals/programming-languages/js.html.md.erb

@@ -120,8 +120,12 @@ Each of the tools previously listed has plugin to help integrate with editors:
 
 Use soft tabs with a two space indent.
 
+If you're using [Prettier](#prettier), this will be set up for you. Otherwise, you may want to configure a [`.editorconfig` file][editorconfig] accordingly.
+
 **Why:** This follows the conventions used within our other projects.
 
+[editorconfig]: https://editorconfig.org/
+
 ## Naming conventions
 
 * Avoid single letter names. Be descriptive with your naming.