Modules for Elvish Shell

docs: Add flake import

Changed files
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README.md
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`flake.nix` supplies a NixOS module that lets you import in the same manner as `epm`
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Input
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```nix
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mellon.url = "git+https://git.sr.ht/~ejri/mellon";
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```
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Use module
```nix
{
...