Add section on plugin installation (#76)

README.md

@@ -8,6 +8,16 @@ Hyperfocus-writing in Vim.
 Best served with [Goyo.vim](https://github.com/junegunn/goyo.vim).
 Works on 256-color terminal or on GVim.
 
+Installation
+------------
+
+Use your favorite plugin manager.
+
+- [vim-plug](https://github.com/junegunn/vim-plug)
+    1. Add `Plug 'junegunn/limelight.vim'` to your Vim configuration file
+    2. Run `:source %`
+    3. Run `:PlugInstall`
+
 Usage
 -----