Update README.md

Author: kojix2

README.md

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 # unicode_plot.cr
 
+[![Test](https://github.com/kojix2/unicode_plot/actions/workflows/test.yml/badge.svg)](https://github.com/kojix2/unicode_plot/actions/workflows/test.yml)
+
 Unicode terminal plots for Crystal — a port of Julia's [UnicodePlots.jl](https://github.com/JuliaPlots/UnicodePlots.jl).
 
 The code was ported from Julia using an AI tool