nix: Add nixfmt package to shell.nix, update readme

nix/README.md

@@ -6,7 +6,7 @@ To build Voxygen, run:
 To build another binary, run:
 `nix build --arg crateName "<binary name here>"`
 
-To enter the development shell, run:
+To enter the development shell (which includes all tools mentioned in this readme), run:
 `nix-shell shell.nix`
 
 ### Managing Cargo.nix
@@ -25,3 +25,10 @@ Enter the development shell in repository root:
 
 To update the dependencies, run:
 `niv update`
+
+### Formatting
+
+Use [nixfmt](https://github.com/serokell/nixfmt) to format files.
+
+To format every file:
+`nixfmt *.nix`

nix/default.nix

@@ -1,6 +1,6 @@
 { crateName ? "veloren-voxygen",
 # `crate2nix` doesn't support profiles in `Cargo.toml`, so default to release. Otherwise bad performance (non-release is built with opt level 0)
-release ? true, sources, nixpkgsSrc }:
+release ? true, sources ? import ./sources.nix { }, nixpkgsSrc ? <nixpkgs> }:
 
 let
   # Check if git-lfs is working.

nix/shell.nix

@@ -9,6 +9,7 @@ in pkgs.mkShell {
     git
     git-lfs
     niv
+    nixfmt
     crate2nix
     rustup
   ];