Update README.md with "maintained by Plotly" badge

Author: gvwilson

README.md

@@ -10,6 +10,12 @@
 
 👉 [Demo source code](https://github.com/plotly/react-plotly.js-demo-app)
 
+<div align="center">
+  <a href="https://dash.plotly.com/project-maintenance">
+    <img src="https://dash.plotly.com/assets/images/maintained-by-plotly.png" width="400px" alt="Maintained by Plotly">
+  </a>
+</div>
+
 ---
 
 ## Contents