message: added activity badge to README
diff: <a href="https://snyk.io/test/npm/remult">
  <img alt="snyk" src="https://snyk.io/test/npm/remult/badge.svg"/>
  </a>
  + <a href="https://github.com/remult/remult/pulse" alt="Activity">
  + <img src="https://img.shields.io/github/commit-activity/m/remult/remult" /></a>
  </div>
  ## What is Remult?
  