Update README.md

Author: mykelk

README.md

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 # DecisionMakingProblems
 
-[![Docs](https://img.shields.io/badge/docs-stable-blue.svg)](https://algorithmsbooks.github.io/DecisionMakingProblems.jl/)
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+[![codecov](https://codecov.io/gh/algorithmsbooks/DecisionMakingProblems.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/algorithmsbooks/DecisionMakingProblems.jl)
+[![Docs](https://img.shields.io/badge/docs-stable-blue.svg)](https://algorithmsbooks.github.io/DecisionMakingProblems.jl/)
 
 This package contains various decision problem environments from [Algorithms for Decision Making](https://algorithmsbook.com/) by Mykel Kochenderfer, Tim Wheeler and Kyle Wray.