Update README.md

Author: ChrisRackauckas

README.md

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 [![Build Status](https://github.com/SciML/RecursiveArrayTools.jl/workflows/CI/badge.svg)](https://github.com/SciML/RecursiveArrayTools.jl/actions?query=workflow%3ACI)
 [![codecov.io](http://codecov.io/github/SciML/RecursiveArrayTools.jl/coverage.svg?branch=master)](http://codecov.io/github/SciML/RecursiveArrayTools.jl?branch=master)
+[![Build status](https://badge.buildkite.com/5f39777d009ce94ef1dcf2a4881c68b9fbcaf6f69f1d8b8df2.svg)](https://buildkite.com/julialang/recursivearraytools-dot-jl)
 
 RecursiveArrayTools.jl is a set of tools for dealing with recursive arrays like
 arrays of arrays. The current functionality includes: