GitHub would be a good place to start, but really it should be a project you've used a fair amount and that you really like. No need to approach them. If you w

GitHub would be a good place to start, but really it should be a project you've used a fair amount and that you really like. No need to approach them. If you want to fill in the gaps in the docs they have, then fork the project, improve the documentation, and send them a pull request. If you want to start a separate project documenting the project, whip something up that's decent but doesn't take you too long and send them a GitHub message. Don't be shy. The sense I get is that people that want to do documentation are extremely hard to come by, so most OSS project leaders would jump for joy if they got a message about someone improving documentation for their project.