Update HACKING.md with add update_settings interface docs.

Above is an example of ```eaf-camera-save-path```, you can customize any variable on elisp side actually, don't need modify python code to customize EAF application!

### Update settings at Python side along with customize option change at Elisp side.

Once you change customize option by ```eaf-setq```, everytime EAF buffer is created, AppBuffer's interface ```update_settings``` will execute.

You can implement your own ```update_settings``` interface, such as, we can write below ```update_settings``` in browser plugin:

```Python

def update_settings(self):

    settings = QWebEngineSettings.globalSettings()

    try:

        settings.setAttribute(QWebEngineSettings.PluginsEnabled, self.emacs_var_dict["eaf-browser-enable-plugin"] == "true")

        settings.setAttribute(QWebEngineSettings.JavascriptEnabled, self.emacs_var_dict["eaf-browser-enable-javascript"] == "true")

    except Exception:

        pass

```

## Todolist

[Some works you can hacking ;)](TODOLIST.md)