Added doi

Author: jagoosw

README.md

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 # CamPyRoS - A 6DOF Rocket Trajectory Simulator
+[![DOI](https://zenodo.org/badge/308847422.svg)](https://zenodo.org/badge/latestdoi/308847422)
 
 CamPyRoS (Cambridge Python Rocketry Simulator) is a Python package which provides fully featured rocket trajectory simulation including features like:
 - 6 degrees of freedom (3 translational, 3 rotational)
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