Update serve.md

docs/src/cmd/serve.md

@@ -46,4 +46,12 @@ You can use `--example {name}` to start a example code.
 ```
 # build `example/test` code
 dioxus serve --exmaple test
-```
+```
+
+## Open Browser
+
+You can add `--open` flag to open system default browser when server startup.
+
+```
+dioxus serve --open
+```