Add local plugin usage to the README

This was added to the README in
466d1839b3, but for some reason the
functionality is still there but it’s no longer documented in the
README.

README.md

@@ -59,6 +59,9 @@ Plug 'tpope/vim-fireplace', { 'for': 'clojure' }
 " Using git URL
 Plug 'https://github.com/junegunn/vim-github-dashboard.git'
 
+" Local plugin
+Plug '/my/local/vim/plugin'
+
 " Plugin options
 Plug 'nsf/gocode', { 'tag': 'v.20150303', 'rtp': 'vim' }