Update README.md with "maintained by community" badge

Author: gvwilson

README.md

@@ -1,3 +1,9 @@
 # plotly.matlab-docs
 
-Documentation for the Plotly MATLAB graphing library.
+Documentation for the Plotly MATLAB graphing library.
+
+<div align="center">
+  <a href="https://dash.plotly.com/project-maintenance">
+    <img src="https://dash.plotly.com/assets/images/maintained-by-community.png" width="600px" alt="Maintained by the Plotly Community">
+  </a>
+</div>