NetlifyCMS Developer Setup: Adding Modular Content Widget

NetlifyCMS Contributor Guide: https://github.com/netlify/netlify-cms/blob/master/CONTRIBUTING.md

Modular Content Widget: https://github.com/netlify/netlify-cms/pull/1169

Installing Yarn: https://yarnpkg.com/lang/en/docs/install/ ... https://www.youtube.com/watch?v=R4rLx6wTqMw