Update build status on README

Author: ranjanan

README.md

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 # Algebraic Multigrid (AMG)
 
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+| **Build Status**|
+|:----------------------------------------------------------------:|
+| [![Build Status](https://github.com/JuliaLinearAlgebra/AlgebraicMultigrid.jl/workflows/CI/badge.svg)](https://github.com/JuliaLinearAlgebra/AlgebraicMultigrid.jl/actions?query=workflow%3ACI)  |
 
 This package lets you solve sparse linear systems using Algebraic Multigrid (AMG). This works especially well for symmetric positive definite matrices.