Add Nix instructions to README

DEPENDENCIES.md

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+# Dependencies
+
+Iced requires some system dependencies to work, and not
+all operating systems come with them installed.
+
+You can follow the provided instructions for your system to
+get them, if your system isn't here, add it!
+
+## NixOS
+
+You can add this `shell.nix` to your project and use it by running `nix-shell`:
+
+```nix
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell rec {
+  buildInputs = with pkgs; [
+    expat
+    fontconfig
+    freetype
+    freetype.dev
+    libGL
+    pkgconfig
+    xorg.libX11
+    xorg.libXcursor
+    xorg.libXi
+    xorg.libXrandr
+  ];
+
+  LD_LIBRARY_PATH =
+    builtins.foldl' (a: b: "${a}:${b}/lib") "${pkgs.vulkan-loader}/lib" buildInputs;
+}
+```