Add Plug install instructions

Prior to this commit it was not obvious that this vim plugin supported
Plug as an installation method.

This commit fixes that by adding a small note to the Installation
section that describes how a user would install this plugin with
Plug.

README.md

@@ -51,11 +51,18 @@ My entire home directory is a git repository, so for me it's simply a case of
 If you're not using pathogen, you can just manually place the files in the
 appropriate places under `~/.vim/`
 
+With [Plug](https://github.com/junegunn/vim-plug)
+
+In your ~/.vimrc (or stdpath('config') . '/init.vim' for Neovim)
+
+    call plug#begin()
+      rodjek/vim-puppet
+    call plug#end()
+
 Testing
 -------
 
 Testing is based on vader.vim testing framework, see:
 <https://github.com/junegunn/vader.vim> . To run full test suit use
 `./test/run-tests.sh`, this will also download vader.vim plugin to project's
 folder.
-