message: docs(readme): add sales pitch
diff: +A Carbon Design System variant that's as easy to use as native HTML elements, with no framework tax, no framework silo.
  +
  <p align="center">
  <a href="https://www.carbondesignsystem.com">
  <img alt="Carbon Design System" src="https://user-images.githubusercontent.com/3901764/57545698-ce5f2380-7320-11e9-8682-903df232d7b0.png" width="100%" />
  