Add build instructions for nix

.gitignore

@@ -12,3 +12,4 @@ local/
 cabal.sandbox.config
 cabal.project.local
 .ghc.environment.*
+result

README.md

@@ -94,6 +94,12 @@ log the size of the input, but _not_ the full input/output of requests.)
     aura -A brittany
     ~~~~
 
+- via `nix`:
+    ~~~.sh
+    nix build  # or 'nix-build'
+    nix-env -i ./result
+    ~~~
+
 # Editor Integration
 
 #### Sublime text

default.nix

@@ -0,0 +1,13 @@
+{ pkgs ? import (fetchGit (import ./pkgs.nix)) {}
+, compiler ? "ghc822"
+}:
+
+pkgs.haskell.packages.${compiler}.developPackage {
+  root = ./.;
+  name = "brittany";
+  overrides = with pkgs.haskell.lib; self: super: {
+  };
+  source-overrides = {
+    ghc-exactprint = "0.5.8.0";
+  };
+}

pkgs.nix

@@ -0,0 +1,5 @@
+{
+  url = "https://github.com/nixos/nixpkgs.git";
+  ref = "release-18.09";
+  rev = "b9fa31cea0e119ecf1867af4944ddc2f7633aacd";
+}