Add LOC badge

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)
+[![Lines of Code](https://img.shields.io/endpoint?url=https%3A%2F%2Ftokei.kojix2.net%2Fbadge%2Fgithub%2Fkojix2%2Funicode_plot%2Flines)](https://tokei.kojix2.net/github/kojix2/unicode_plot)
 
 Unicode terminal plots for Crystal — a port of Julia's [UnicodePlots.jl](https://github.com/JuliaPlots/UnicodePlots.jl).