code: async function copyMandatoryFiles () {
    const buildDir = createAndGetBuildFolder()
  
    for (const f of assetsToCopy) {
      const filepath = path.resolve(f)
  
      // Get filename and dirname from the provided path.
      const filename = path.basename(filepath)
      const dirname = path.basename(path.dirname(filepath))
  
      // Create the directory for this file if needed.
      // ie. css/master.css needs a `css` directory.
      const targetDir = path.join(buildDir, dirname)
      sfs.createDir(targetDir)
  
      // And copy the file.
      const targetPath = path.join(targetDir, filename)
  
      console.log('- ' + targetPath)
      try {
        await writeFilePromised(targetPath, await readFilePromised(filepath))
      } catch (e) {
        const msg = chalk.dim(`(${e.message})`)
        console.error(`There was an error while copying ${chalk.bold(filename)}. ${msg}`)
      }
    }
  }
docs: Add css and any mandatory files specified to the build directory