add doc build badge

Author: inkydragon

README.md

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 # LinearAlgebra
 
+[![Doc Build](https://github.com/JuliaLang/LinearAlgebra.jl/actions/workflows/ci.yml/badge.svg)](https://github.com/JuliaLang/LinearAlgebra.jl/actions/workflows/ci.yml)
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 This package is part of the Julia standard library (stdlib).
 
 `LinearAlgebra.jl` provides functionality for performing linear algebra operations in Julia.