Update README.md with "maintained by Plotly" badge

Author: gvwilson

README.md

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 # Contributing to Plotly's Graphing Libraries Documentation
 
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+ <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>
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 ## Repo Overview
 
 Plotly welcomes contributions to its [open-source graphing libraries documentation](https://plotly.com/graphing-libraries) from its community of users.