Add note to Laravel Mix instructions warning about Sass issue

docs/source/docs/installation.blade.md

@@ -162,3 +162,15 @@ mix.less('source/_assets/less/main.less', 'source/css')
     ]
   })
 ```
+
+**Note for Sass users:** Due to [an unresolved issue](https://github.com/bholloway/resolve-url-loader/issues/28) with one of Mix's dependencies, to use Sass with Tailwind you'll need to disable `processCssUrls`:
+
+```js
+var tailwindcss = require('tailwindcss');
+
+mix.sass('resources/assets/sass/app.scss', 'public/css')
+   .options({
+      processCssUrls: false,
+      postCss: [ tailwindcss('path/to/config.js') ],
+   });
+```