Update README.md with nixpkgs instructions (#128)

README.md

@@ -46,8 +46,12 @@ Options:
 There are a couple of ways to install Caligula.
 
 - **Binary release:** You can download pre-built binaries from [the latest Github release](https://github.com/ifd3f/caligula/releases/latest).
-- **Arch Linux:** You can install from the [official repository](https://archlinux.org/packages/extra/x86_64/caligula) using pacman. We also automatically publish binaries to [caligula-bin on the AUR](https://aur.archlinux.org/packages/caligula-bin) with every release.
-- **Nix Package Manager:** If your system is flake-enabled, you can fetch Caligula from the `github:ifd3f/caligula` flake (i.e. with `nix run github:ifd3f/caligula`)
+- **Arch Linux:** 
+  - [Official repository](https://archlinux.org/packages/extra/x86_64/caligula): `pacman -S caligula`
+  - [caligula-bin on the AUR](https://aur.archlinux.org/packages/caligula-bin): We also automatically publish binaries with every release.
+- **Nix:**
+  - [Nixpkgs](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/ca/caligula/package.nix): `nix-env -i caligula`
+  - Repository flake: If your system is flake-enabled, you can use `github:ifd3f/caligula` for bleeding-edge changes.
 - **Build from source:** This is a relatively standard cargo project so you should be able to just `git clone` and `cargo build --release` it.
 
 ### Platform support matrix