Note that frontmatter is only optional for _posts

I was just talking with @benbalter and it sounds like you can only omit YAML frontmatter from Markdown files under the `_posts` directory—if you omit it in a Markdown file anywhere else, the file will be processed as if it were static (HTML) content. That may or may not be true for collections, as well.

This pull request is just a starting point to clarify [the documentation on jekyllrb.com](http://jekyllrb.com/docs/frontmatter/).