Update README.md with "maintained by Plotly" badge

Author: gvwilson

README.md

@@ -4,6 +4,12 @@ Authentication for apps deployed to [Dash Enterprise](https://plotly.com/dash)
 
 Learn more at https://dash.plotly.com/dash-enterprise/app-authentication
 
+<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>
+
 ```py
 import dash_enterprise_auth as auth