Add mention of publicPath to doc so people who know webpack can find …

…this

docs/docs/path-prefix.md

@@ -11,7 +11,7 @@ Each of these sites need a prefix added to all paths on the site. So a link to
 `/my-sweet-blog-post/` should be rewritten to `/blog/my-sweet-blog-post`.
 
 In addition links to various resources (JavaScript, images, CSS) need the same
-prefix added.
+prefix added (this is accomplished by setting the `publicPath` in webpack).
 
 Luckily, for most sites, this work can be offloaded to Gatsby. Using
 [Gatsby's Link component](/packages/gatsby/) for internal links ensures those links