added explaination for ~ syntax

Author: sokra

README.md

@@ -13,6 +13,10 @@ css code will be minimized if specified by the module system.
 Good loaders for requiring your assets are the [file-loader](https://github.com/webpack/file-loader)
 and the [url-loader](https://github.com/webpack/url-loader) which you should specify in your config (see below).
 
+To be combatible to existing css files:
+* `url(image.png)` => `require("./image.png")`
+* `url(~module/image.png)` => `require("module/image.png")`
+
 ### Example config
 
 This webpack config can load css files, embed small png images as Data Urls and jpg images as files.