[Phase 2] update readme to clarify how CSS should be written for this repo

[MelSumner]

update the readme to clarify what style of CSS should be written for this project.

[mansona]

We should update https://github.com/ember-learn/ember-styleguide/blob/website-redesign-rfc/docs/css/overview.md to describe requirements for writing CSS in this project (mobile-first, relative units) and refer to this file from the readme

(note this is on the redesign branch 👍 )