Replace the README badge by the CodeCov one (#392)

Author: theogf

README.md

@@ -1,7 +1,7 @@
 # KernelFunctions.jl
 
 ![CI](https://github.com/JuliaGaussianProcesses/KernelFunctions.jl/workflows/CI/badge.svg?branch=master)
-[![Coverage Status](https://coveralls.io/repos/github/JuliaGaussianProcesses/KernelFunctions.jl/badge.svg?branch=master)](https://coveralls.io/github/JuliaGaussianProcesses/KernelFunctions.jl?branch=master)
+[![codecov](https://codecov.io/gh/JuliaGaussianProcesses/KernelFunctions.jl/branch/master/graph/badge.svg?token=rmDh3gb7hN)](https://codecov.io/gh/JuliaGaussianProcesses/KernelFunctions.jl)
 [![Documentation (stable)](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliagaussianprocesses.github.io/KernelFunctions.jl/stable)
 [![Documentation (latest)](https://img.shields.io/badge/docs-dev-blue.svg)](https://juliagaussianprocesses.github.io/KernelFunctions.jl/dev)
 [![ColPrac: Contributor's Guide on Collaborative Practices for Community Packages](https://img.shields.io/badge/ColPrac-Contributor's%20Guide-blueviolet)](https://github.com/SciML/ColPrac)