Add vim-pandoc plugin for pandoc & markdown.

I prefer pandoc-flavored Markdown to all other forms of Markdown.
Note that vim-pandoc and vim-markdown conflict in terms of syntax
highlighting. I'd rather have pandoc functionality than generic
Markdown, so install this plugin instead of vim-markdown.
Add placeholder README to autoload directory.

Add README to ensure autoload directory exists, and to remind users
of the purpose of the directory.