Add ``express static`` in router

[sigristarisa]

• use express.static to serve static files from a directory

  app.use(express.static('public')); // serve all static files in the public directory
  app.use('/assets', express.static('assets'));
  app.use('/users', userRouter)

• possilbe to use a URL like normal

  http://localhost:4000/assets/img_001.png