Documentation for including JavaScript files in the output

[nknapp]

It must still be documented, how JavaScript can be included into the bootprint output.
Pointers are in the customize-engine-uglify project, especially the example

merge({
    uglify: {
      files: {
        'a-browser-lib.js': require.resolve('./module1/a-browser-lib.js'),
        'another-browser-lib.js': require.resolve('./module1/another-browser-lib.js')
      },
      dependencies: {
        'a-browser-lib.js': [ 'another-browser-lib.js' ]
      }
    }
  })

and the configuration schema for this engine