Clarify uglify-js peer dependency and #harmony branch + migrate to Yarn
Clarify uglify-js peer dependency and #harmony branch + migrate to Yarn

Make it clear for the user on how to enable minification for ES6 code.

Also migrate the examples from npm to Yarn, as Yarn is superior to npm in many ways (speed, determinism, etc.) and is gradually becoming the new industry leader.
webpack-contrib/uglifyjs-webpack-plugin,michael-ciniawsky/uglifyjs-webpack-plugin