Update README.md

Author: andreasnoack

README.md

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 [![CI](https://github.com/JuliaStats/KernelDensity.jl/actions/workflows/CI.yml/badge.svg)](https://github.com/JuliaStats/KernelDensity.jl/actions/workflows/CI.yml)
 [![Coverage Status](https://coveralls.io/repos/github/JuliaStats/KernelDensity.jl/badge.svg)](https://coveralls.io/github/JuliaStats/KernelDensity.jl)
+[![Stable Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliastats.org/KernelDensity.jl/stable/)
 
 Kernel density estimators for Julia.