Update README to encourage use of 'npm start'

[kmturley]

Node and JavaScript projects using npm have a convention of npm install for installation, npm start for running and npm run build for production builds. Using these means a developer does not need to remember custom commands per project.

Your project already supports all of these commands, but in the README it directs users to use ng serve and ng build.

Update lines to be:

Run `npm start` for a dev server. Navigate to `http://localhost:4200/`. The app will automatically reload if you change any of the source files.

Run `npm run build` to build the project. The build artifacts will be stored in the `dist/` directory. It uses a `-prod` flag for a production build.