`npm run start:webpack` does not work as described in README

hey, I noticed that the Vaadin Connect starter app (https://github.com/vaadin/base-starter-connect) gives an error npm ERR! missing script: start:webpack if you try to run the processes separately with npm run start:webpack.

It looks like the README is out of date. You either have to run npm start (which runs webpack, backend and apibrowser) or write your own start:webpack script if you want to run only webpack

The way to do that manually would be ./node_modules/.bin/webpack --mode development --watch

DoD:

• README contains correct instructions on how to start only the front-end in the dev mode
• (it may require adding a new script into @vaadin/connect-scripts)