Document new installation option (#9)

Hey team-

As of NixOS/nixpkgs#114036, folks will be able to install `stan-vim` from [nixpkgs](https://nixos.org/). I personally use a tool based on nix -- [Home Manager](https://github.com/nix-community/home-manager) -- to manage my vim plugins, which makes it tricky to _also_ use a purpose-built vim package manager or manual installation. This tweak to the README simply documents that new installation option for the small community of other people who manage their vim this way.

Thanks for your consideration!

Author: wbadart

README.md

@@ -14,6 +14,7 @@ A Vim plugin for the [Stan probabilistic programming language](https://mc-stan.o
 - [Installation](#installation)
   - [Vundle, NeoBundle and VimPlug](#vundle-neobundle-and-vimplug)
   - [Pathogen](#pathogen)
+  - [Home Manager](#home-manager)
   - [Manual Installation](#manual-installation)
 - [Documentation](#documentation)
 - [Additional Screenshots](#additional-screenshots)
@@ -58,6 +59,21 @@ cd ~/.vim/bundle
 git clone https://github.com/eigenfoo/stan-vim
 ```
 
+### Home Manager
+
+Nix users who manage their Vim plugins with Home Manager can grab `stan-vim` from nixpkgs:
+
+```nix
+{ pkgs, ... }:
+{
+  # or programs.neovim.plugins
+  programs.vim.plugins = with pkgs.vimPlugins; [
+    stan-vim
+    # ...
+  ];
+}
+```
+
 ### Manual Installation
 
 Copy all files into your `~/.vim` directory.