Clarify postcss-url usage docs

Resolves #521

README.md

@@ -20,10 +20,9 @@ You can also provide manually multiples paths where to look at.
 - **This plugin should probably be used as the first plugin of your list.
 This way, other plugins will work on the AST as if there were only a single file
 to process, and will probably work as you can expect**.
-- This plugin works great with
-[postcss-url](https://github.com/postcss/postcss-url) plugin,
-which will allow you to adjust assets `url()` (or even inline them) after
-inlining imported files.
+- Running [postcss-url](https://github.com/postcss/postcss-url) after
+postcss-import in your plugin chain will allow you to adjust assets `url()` (or
+even inline them) after inlining imported files.
 - In order to optimize output, **this plugin will only import a file once** on
 a given scope (root, media query...).
 Tests are made from the path & the content of imported files (using a hash