message: Update README.md
  added npm downloads badge
diff: <a href="https://raw.githubusercontent.com/remult/remult/master/LICENSE">
  <img alt="GitHub license" src="https://img.shields.io/badge/license-MIT-blue.svg">
  </a>
  - <a href="https://badge.fury.io/js/remult">
  + <a href="https://www.npmjs.com/package/remult">
  <img alt="npm version" src="https://badge.fury.io/js/remult.svg">
  </a>
  + <a href="https://www.npmjs.com/package/remult">
  + <img alt="npm downloads" src="https://img.shields.io/npm/dm/remult">
  + </a>
  </div>
  ## What is Remult?
  