RMarkdown preamble

• Every RMarkdown document starts with a preamble block, technically referred to as the “YAML header”, which contains general document information.
• The block starts with a line of three dashes, and ends with another line of three dashes.
• The contents of the block must follow a very specific syntax: Usually a keyword, followed by a colon, followed by a “value” often enclosed in quotation marks.
• The most common keywords are title, author and date, with obvious meanings.
• The output keyword specifies the kind of output document to produce. We will be almost exclusively work with html_document but there are many other options.