Fix nixpkgs link and add flake-based command

Signed-off-by: Nick Gerace <[email protected]>

README.md

@@ -122,12 +122,18 @@ pacman -S gfold
 
 ### Nix and NixOS
 
-You can install `gfold` from [nixpkgs](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/version-management/git-and-tools/gfold/default.nix):
+You can install `gfold` from [nixpkgs](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/version-management/gfold/default.nix):
 
 ```shell
 nix-env --install gfold
 ```
 
+If you are using [flakes](https://nixos.wiki/wiki/Flakes), you can install using the `nix` command directly.
+
+```shell
+nix profile install "nixpkgs#gfold"
+```
+
 ### Cargo Install
 
 You can use [cargo](https://crates.io) to install the [crate](https://crates.io/crates/gfold) on almost any platform.