Update README.md

Author: RoryBarnes

README.md

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   <img src="https://img.shields.io/badge/Unit%20Tests-11-darkblue.svg"/></a>
   <a href="https://github.com/VirtualPlanetaryLaboratory/vplot/actions/workflows/tests.yml">
     <img src="https://github.com/VirtualPlanetaryLaboratory/vplot/actions/workflows/tests.yml/badge.svg"/>
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+  <img src="https://img.shields.io/badge/Python-3.6%20--%203.12-7d93c7.svg">
 </p>
 <br>
 VPLot simplifies plotting [VPLanet](https://github.com/VirtualPlanetaryLaboratory/vplanet) data through a command line option that instantly generates plots of a single simulation, as well as a Python module to generate consistent and accessible figures for your publications.