Update README.md with "maintained by Plotly" badge

Author: gvwilson

README.md

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 # Paketo Buildpack for CPython Cloud Native
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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">
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 The CPython Buildpack provides CPython (reference implementation of Python) 3.
 The buildpack installs CPython onto the `$PATH` which makes it available for
 subsequent buildpacks and in the final running container. It also sets the