message: added snyk badge to README
diff: <a href="https://www.npmjs.com/package/remult">
  <img alt="npm downloads" src="https://img.shields.io/npm/dm/remult">
  </a>
  + <a href="https://snyk.io/test/npm/remult">
  + <img alt="snyk" src="https://snyk.io/test/npm/remult/badge.svg"/>
  + </a>
  </div>
  ## What is Remult?
  