1.  Modify README.md and other files to reflect use of GitHub Import.
2.  Modified URLs for shared files in `_include` files to get the right images and CSS.

This turned up #137: the CSS files for the workshop template, the main
website, and the lesson repository are all out of step and need to be
synchronized.  Once that is done, the URLs in the
`_includes/header.html` file should be updated