Fix #397: clarify in docs that GitHub is the default plugin source

plug.vim

@@ -11,9 +11,13 @@
 "   call plug#begin('~/.vim/plugged')
 "
 "   " Make sure you use single quotes
-"   Plug 'junegunn/seoul256.vim'
+"
+"   " Shorthand notation; fetches https://github.com/junegunn/vim-easy-align
 "   Plug 'junegunn/vim-easy-align'
 "
+"   " Any valid git URL is allowed
+"   Plug 'https://github.com/junegunn/vim-github-dashboard.git'
+"
 "   " Group dependencies, vim-snippets depends on ultisnips
 "   Plug 'SirVer/ultisnips' | Plug 'honza/vim-snippets'
 "
@@ -21,9 +25,6 @@
 "   Plug 'scrooloose/nerdtree', { 'on':  'NERDTreeToggle' }
 "   Plug 'tpope/vim-fireplace', { 'for': 'clojure' }
 "
-"   " Using git URL
-"   Plug 'https://github.com/junegunn/vim-github-dashboard.git'
-"
 "   " Using a non-master branch
 "   Plug 'rdnetto/YCM-Generator', { 'branch': 'stable' }
 "
@@ -40,7 +41,21 @@
 "   call plug#end()
 "
 " Then reload .vimrc and :PlugInstall to install plugins.
-" Visit https://github.com/junegunn/vim-plug for more information.
+"
+" Plug options:
+"
+"| Option                  | Description                                      |
+"| ----------------------- | ------------------------------------------------ |
+"| `branch`/`tag`/`commit` | Branch/tag/commit of the repository to use       |
+"| `rtp`                   | Subdirectory that contains Vim plugin            |
+"| `dir`                   | Custom directory for the plugin                  |
+"| `as`                    | Use different name for the plugin                |
+"| `do`                    | Post-update hook (string or funcref)             |
+"| `on`                    | On-demand loading: Commands or `<Plug>`-mappings |
+"| `for`                   | On-demand loading: File types                    |
+"| `frozen`                | Do not update unless explicitly specified        |
+"
+" More information: https://github.com/junegunn/vim-plug
 "
 "
 " Copyright (c) 2015 Junegunn Choi