Fix README example: wrap config with useMemo()!

I mean section
https://github.com/jodit/jodit-react#1-require-and-use-jodit-react-component-inside-your-application

Replace

const Example = ({}) => {

const config = {
    readonly: false // all options from https://xdsoft.net/jodit/doc/
};

with

const Example = ({placeholder}) => {

const config: JoditEditorOptions = useMemo(() => ({
    // all options from https://xdsoft.net/jodit/doc/
    readonly: false 
    placeholder: placeholder || 'Start typing here...', 
}), [placeholder])

Without this fix editor may lose focus every time you change it's value.

See https://github.com/jodit/jodit-react/issues/43#issuecomment-580152633

Start with the README section linked in the issue, titled “Require and use Jodit React component inside your application,” and compare its Example component and config snippet with the requested usage. Update the example so its configuration is stable and supports the placeholder shown in the issue; done means the README reflects the requested example without the focus-loss warning.