Improve docs about differences with Webpack

[piotrwitek]

Add section in docs about differences of this workflow compared to Webpack.

Module Bundler vs. ES Module Loader for Browsers

[piotrwitek]

JSPM with SystemJS aims to enable the workflow to load ES modules directly by the browser with "ES Modules" support (without the bundling process), or falling back to a SystemJS loader in browsers without such support.