Add deployment section to README.md

Add a short section with infos about building and deploying. Nothing detailed, but just a reminder that

npm run dist creates the files in the ./dist directory
these files can be hosted statically or integrated into whatever production servers and server-side frameworks the users deploy to
npm run server:dist makes use of the webpack-dev-server and (because of that) is only meant for testing and not running in production

react-webpack-generators / generator-react-webpack