Add notes on discussion on polyfills in presets

[hzoo]

Ref https://github.com/bettiolo/babel-preset-es2017/issues/42

Idea is to include the relevant polyfills in the yearly presets.

In node: add babel-preset-es2017 to the preset to get transpilation and polyfill. In the browser: import “babel-preset-es2017/polyfill"

• concerns about size and different versions of polyfill/core-js

and/or polyfill by year

import "babel-polyfill/es2015"