add codecov badge

Author: ablaom

README.md

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 # MLJModels
 
 [![Build Status](https://github.com/JuliaAI/MLJModels.jl/workflows/CI/badge.svg)](https://github.com/JuliaAI/MLJModels.jl/actions)
+[![codecov](https://codecov.io/gh/JuliaAI/MLJModels.jl/graph/badge.svg?token=KgarnnCc0K)](https://codecov.io/gh/JuliaAI/MLJModels.jl)
 
 Repository of the "built-in" models available for use in the
 [MLJ](https://github.com/JuliaAI/MLJ.jl) MLJ machine