We use Github Wikis for anything not sensitive. If we need to share files that have sensitive info, we PGP encrypt them and send them to each other through our own server.
Founder & CEO - Coinkite
We are heavy users of GitHub Wikis. Everything from mission statements for certain engineering teams, to planning and collaboration documents, along with informal PRDs, are all in our GitHub Wikis. And everything is also linked off of Wizeline.
Founder & CEO - Wizeline
GitHub Wikis are a place in your repository where you can share long-form content about your project, such as how to use it, how it's been designed, manifestos on its core principles, and so on.
Visit the website: https://help.github.com/articles/about-github-wikis/