readme: add total npm downloads badge (gatsbyjs#9926)

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The total figure is much higher (6 million) than the monthly rate and quite a lot more impactful. So, it should make for a better listing on the readme :)

README.md

@@ -27,6 +27,9 @@
   <a href="https://npmcharts.com/compare/gatsby?minimal=true">
     <img src="https://img.shields.io/npm/dm/gatsby.svg" alt="Downloads per month on npm." />
   </a>
+  <a href="https://npmcharts.com/compare/gatsby?minimal=true">
+    <img src="https://img.shields.io/npm/dt/gatsby.svg" alt="Total downloads on npm." />
+  </a>
   <a href="https://gatsbyjs.org/docs/how-to-contribute">
     <img src="https://img.shields.io/badge/PRs-welcome-brightgreen.svg" alt="PRs welcome!" />
   </a>