http://ycombinator.com/newsfaq.html

In there it says:

What kind of formatting can you use in comments?

http://news.ycombinator.com/formatdoc

On that page it says:

    Blank lines separate paragraphs.

    Text after a blank line that is indented by two or
    more spaces is reproduced verbatim. (This is intended
    for code.)

    Text surrounded by asterisks is italicized, if the
    character after the first asterisk isn't whitespace.

    Urls become links, except in the text field of a
    submission.

    # like this