Web: doc for debugging (#632)

docs/web.md

@@ -5,3 +5,25 @@
 Then go to [http://localhost:8080/vtm-web-app](http://localhost:8080/vtm-web-app) in the web browser to see the map.
 
 Hold right mouse button to change view direction.
+
+#### Debugging GWT app
+
+Using GWT SuperDevMode is the recommended way for development. Debugging in IDE is often not possible.
+
+For an introduction see [GWT](http://www.gwtproject.org/articles/superdevmode.html) and [libGDX](http://www.badlogicgames.com/wordpress/?p=3073) documentations.
+
+- Serve the website as usual with `./gradlew :vtm-web-app:farmRun` command.
+
+- The codeserver must be executed on another shell.
+```bash
+export _JAVA_OPTIONS="-Xmx1024m"
+./gradlew :vtm-web-app:gwtSuperDev
+```
+
+- Open the link of codeserver: `The code server is ready at http://127.0.0.1:xxxx/`
+
+- Drag the two bookmarklets to your browser's bookmark bar.
+
+- Visit your [web page](http://localhost:8080/vtm-web-app) and click **Dev Mode On** to start development mode.
+
+- Press **F12** to open the developer tools of your browser. Now you can debug your code under **Sources**.