Yeah, in terms of gaining users I think that learning a little bit of basic markdown formatting is very worthwhile. It only takes a few minutes to learn the important stuff (headings, bullets, code formatting, hyperlinks), and the knowledge will continue to pay off on future open source projects. (I am a recent convert from plaintext readme files, and am happy with the decision.)

Visually, having a decent README sets the project apart, and makes it look more "put together." Anecdotally, I can say that when I'm looking for a library to solve a particular problem, I'm probably going to stick around and read more about the repo that has beautiful markdown documentation, all else being equal. Maybe I'm just vain, though... :)

You just have to name your file README.md :)