improve js build doc (#43)

webiojs/README.md

@@ -4,11 +4,13 @@
 
 ```bash
 npm install
-DEV=1 gulp
+gulp
 ```
 
 ## Use built js
 
 ```bash
 cp dist/pywebio.min.* ../pywebio/html/js
-```
+```
+
+By default, PyWebIO uses CDN for frontend resources. For local dev build, you also need to set `cdn=False` in `start_server()` or `webio_handler()` in your PyWebIO app to specify the use of the local built js file.