docs: add status shield [skip ci]

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README.md
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[![Check and Build configuration](https://github.com/soopyc/nix-on-koumakan/actions/workflows/build.yaml/badge.svg?branch=main)](https://github.com/soopyc/nix-on-koumakan/actions/workflows/build.yaml)
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# Cow
![a yak on some grass](docs/quaritsch-photography-1_6rJHQ2Gmw-unsplash.jpg)