docs: update sass-loader config for 8.0.0 (#511)

Author: tychenjiajun

docs/guide/README.md

@@ -199,11 +199,24 @@ Webpack example:
 
 Vue config example:
 ```javascript
+css: {
+  loaderOptions: {
+    sass: {
+      sassOptions: {
+        includePaths: ["./node_modules"]
+      }
+    }
+  }
+}
+```
+
+For `sass-loader` before `8.0.0`, use the following: 
+```
 css: {
   loaderOptions: {
     sass: {
       includePaths: ['./node_modules']
     }
   }
 }
-```
+```