docs: add readme

README.md

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+# [Nushell](https://www.nushell.sh/) image
+
+This repository builds plain OCI images from the [Nushell's GitHub releases](https://github.com/nushell/nushell/releases).
+
+## Details
+
+-   The images are built once per week on Sunday from the `x86_64-unknown-linux-gnu` version.
+-   Every image is tagged with the corresponding Nushell version, but the latest version also gets the `latest` tag (beware of breaking changes).
+-   Each image includes the full contents of the release `.tar.gz` in the `/nu/` directory.
+-   The images are built with a Nushell script: [`build.nu`](./build.nu).
+-   The images are signed with Sigstore's [cosign](https://github.com/sigstore/cosign) and can be verified with `cosign verify --key cosign.pub ghcr.io/blue-build/nushell-image`.
+
+## How to use
+
+```containerfile
+FROM ghcr.io/blue-build/nushell-image:0.98.0 as nushell
+
+FROM fedora:40
+
+COPY --from=nushell /nu/nu /usr/bin/nu
+
+RUN nu --version && nu -c "ls | sort-by size"
+```