Add documentation for running gh-pages site locally

I spent a little bit of time trying to figure out how to run the gh-pages site locally, so I figured this documentation might save people some time.

One of the requirements of getting the instructions working was renaming `./Gemfiile` to `./Gemfile`. I figured it was a typo.

I've also chosen to include the `./Gemfile.lock` as per [this answer on stackoverflow](http://stackoverflow.com/a/4151540/534640). I don't know how this will conflict with automated deployment of the website though, so input is welcome.