I like to add the documentation directly to my repository on GitHub. I find it is the easiest way to make sure people get the documentation alongside the code, and that it also get versioned (and tagged!) alongside (which would be much harder to achieve with GitHub submodules, IMHO).
Be that as it may, I find time and time again that people have a hard time searching the documentation. So I have started to add this to my README.md file to help them get around that problem:
[Search the documentation](../../search?l=markdown&q=&type=Code)
This will present the user with a screen from where they can search just the Markdown documents, with the text cursor already focused on the search box.
Of course, this works well for me because I document in Markdown format, but you could adapt this example to work with other formats well supported by GitHub's web interface.