add codecov badge

Author: ufechner7

README.md

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-[![Build Status](https://github.com/Albatross-Kite-Transport/VortexStepMethod.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/Albatross-Kite-Transport/VortexStepMethod.jl/actions/workflows/CI.yml?query=branch%3Amain)
+[![Build Status](https://github.com/Albatross-Kite-Transport/VortexStepMethod.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/Albatross-Kite-Transport/VortexStepMethod.jl/actions/workflows/CI.yml?query=branch%3Amain) [![Coverage](https://codecov.io/gh/Albatross-Kite-Transport/VortexStepMethod.jl/branch/main/graph/badge.svg)](https://codecov.io/gh/Albatross-Kite-Transport/VortexStepMethod.jl)
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 # Simulation of a Kite using the Vortex Step Method