Update documentation to cover github exporting.

Prior to this commit, the documentation for this project didn't contain
any details or examples on how to export from a Github repository.  This
made it practically unusable from an end-user's view.

This commit adds documentation on how to export from Github, so that
users can begin to use this feature.  This involves updating the README,
updating the help text printed out for the --help option, and updating
the "usage" error message printed when incorrect arguments are provided.

Issue #7