add codecov badge

README.md

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 [![](https://img.shields.io/badge/docs-stable-blue.svg)](https://putianyi889.github.io/SwapSort.jl/stable)
 [![](https://img.shields.io/badge/docs-dev-blue.svg)](https://putianyi889.github.io/SwapSort.jl/dev)
 [![Aqua QA](https://raw.githubusercontent.com/JuliaTesting/Aqua.jl/master/badge.svg)](https://github.com/JuliaTesting/Aqua.jl)
+[![codecov](https://codecov.io/gh/putianyi889/SwapSort.jl/branch/master/graph/badge.svg?label=codecov)](https://codecov.io/gh/putianyi889/SwapSort.jl)
 
 This package is intended to be a successor of [SortingNetworks.jl](https://github.com/JeffreySarnoff/SortingNetworks.jl). This package relies on the existing [sorters](./src/Sorters) which are currently copied from [SorterHunter](https://github.com/bertdobbelaere/SorterHunter), a C++ project. For a visualization of the algorithms, see [here](https://bertdobbelaere.github.io/sorting_networks.html).