docs: install instructions for vim-plug (#231)

Co-authored-by: Mitchell Hanberg <mitch@mitchellhanberg.com>

Author: Bastes

README.md

@@ -68,6 +68,13 @@ Requires 0.8
 use({ "elixir-tools/elixir-tools.nvim", tag = "stable", requires = { "nvim-lua/plenary.nvim" }})
 ```
 
+## vim-plug
+
+```vimscript
+Plug "nvim-lua/plenary.nvim"
+Plug "elixir-tools/elixir-tools.nvim", { "tag": "stable" }
+```
+
 # Getting Started
 
 ## Minimal Setup