doc/README.md: Add more links

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You can find the [rendered documentation for Nixpkgs `unstable` on nixos.org](https://nixos.org/manual/nixpkgs/unstable/).
If you want to contribute to the documentation, [here's how to do it](https://nixos.org/manual/nixpkgs/unstable/#chap-contributing).
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You can find the [rendered documentation for Nixpkgs `unstable` on nixos.org](https://nixos.org/manual/nixpkgs/unstable/).
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[Docs for Nixpkgs stable](https://nixos.org/manual/nixpkgs/stable/) are also available.
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If you want to contribute to the documentation, [here's how to do it](https://nixos.org/manual/nixpkgs/unstable/#chap-contributing).
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If you're only getting started with Nix, go to [nixos.org/learn](https://nixos.org/learn).