Update README.md with "maintained by community" badge

Author: gvwilson

README.md

@@ -2,3 +2,9 @@
 
 The package that defines base `Component` and `Resource` structure for `Dash.jl`
 and Dash components packages. Not intended for direct use.
+
+<div align="center">
+  <a href="https://dash.plotly.com/project-maintenance">
+    <img src="https://dash.plotly.com/assets/images/maintained-by-community.png" width="400px" alt="Maintained by the Plotly Community">
+  </a>
+</div>