Update README.md

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Author: Cody-G

README.md

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 # ImagineInterface
 [![Build Status](https://travis-ci.org/HolyLab/ImagineInterface.svg?branch=master)](https://travis-ci.org/HolyLab/ImagineInterface)
 [![Build status](https://ci.appveyor.com/api/projects/status/obywii010u9stx5f/branch/master?svg=true)](https://ci.appveyor.com/project/Cody-G/imagineinterface/branch/master)
+[![codecov](https://codecov.io/gh/HolyLab/ImagineInterface/branch/master/graph/badge.svg)](https://codecov.io/gh/HolyLab/ImagineInterface)
 
 Read and write Imagine analog and digital recordings and commands