Add install steps for vim-plug in .vimrc/init.vim (#12)

Author: jc00ke

README.md

@@ -4,6 +4,8 @@ Elixir language server extension based on [elixir-ls](https://github.com/elixir-
 
 ## Install
 
+### CocInstall
+
 1. Make sure you've got `elixir` and `mix` available in `$PATH`.
 
 2. Inside (neo)vim run this command:
@@ -12,6 +14,16 @@ Elixir language server extension based on [elixir-ls](https://github.com/elixir-
 :CocInstall coc-elixir
 ```
 
+### vim-plug
+
+1. Make sure you've got `elixir`, `mix` and `yarn` available in `$PATH`.
+
+2. Inside your `plug#begin/end` block in your `.vimrc` (vim) or `init.vim` (neovim) file, after `coc.nvim` add:
+
+```
+Plug 'amiralies/coc-elixir', {'do': 'yarn install --frozen-lockfile && yarn run build'}
+```
+
 ## Features
 - Go to definition support
 - Code completion