The thing that gets me is how many side projects and indie products only exist on Twitter or a GitHub readme. Like you built something cool but I have to scroll through your tweet history to understand what it does? Even a single page with a clear description, maybe a screenshot, and a link to try it would go so far. I recently launched a Chrome extension and the number of people who told me they almost did not try it because there was not enough info upfront was a wake up call.
What's wrong with a README? It's as close to a website as one can get within a self-contained repository, and is most likely to be kept up to date with any given version of the project. If there is one document available anywhere that describes a project, I would hope it is the README.