Update README.md

Author: findmyway

README.md

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 # ReinforcementLearningCore
 
 [![Build Status](https://travis-ci.com/JuliaReinforcementLearning/ReinforcementLearningCore.jl.svg?branch=master)](https://travis-ci.com/JuliaReinforcementLearning/ReinforcementLearningCore.jl)
+[![CodeCoverage](https://codecov.io/gh/JuliaReinforcementLearning/ReinforcementLearningCore.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaReinforcementLearning/ReinforcementLearningCore.jl)
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+This package is the core component of [ReinforcementLearning.jl](https://github.com/JuliaReinforcementLearning/ReinforcementLearning.jl). It provides some typical implementations of the interfaces defined in [ReinforcementLearningBase.jl](https://github.com/JuliaReinforcementLearning/ReinforcementLearningBase.jl).