Update README.md with "maintained by community" badge

Author: gvwilson

README.md

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 Documentation for the Plotly Julia graphing library.
 
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+  <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>
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 ## Developer Setup