Provide help for missing repo

[peterdesmet]

Not Installed: No such file or directory - git
  Liquid Exception: No repo name found. Specify using PAGES_REPO_NWO environment variables, 'repository' in your configuration, or set up an 'origin' git remote pointing to your [github.com](http://github.com/) repository. in /_layouts/base.html
             ERROR: YOUR SITE COULD NOT BE BUILT:
                    ------------------------------------
                    No repo name found. Specify using PAGES_REPO_NWO environment variables, 'repository' in your configuration, or set up an 'origin' git remote pointing to your [github.com](http://github.com/) repository.

This can be solved by adding the following to _config.yml:

repository: user/repo