Add a flake.nix (#72)

This will make it far easier to get started with eframe/egui. Simply
running the following command will (hopefully) set up a working
environment: `nix --experimental-features 'nix-command flakes' develop`
(if Nix is installed)

flake.nix

@@ -0,0 +1,45 @@
+{
+  description = "eframe devShell";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    rust-overlay.url = "github:oxalica/rust-overlay";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+      in with pkgs; {
+        devShells.default = mkShell rec {
+          buildInputs = [
+            # Rust
+            rust-bin.stable.latest.default
+            trunk
+
+            # misc. libraries
+            openssl
+            pkgconfig
+
+            # GUI libs
+            libxkbcommon
+            libGL
+            fontconfig
+
+            # wayland libraries
+            wayland
+
+            # x11 libraries
+            xorg.libXcursor
+            xorg.libXrandr
+            xorg.libXi
+            xorg.libX11
+
+          ];
+
+          LD_LIBRARY_PATH = "${lib.makeLibraryPath buildInputs}";
+        };
+      });
+}