Add a README file

[cgay]

Document how to serve the files, since just starting an HTTP server with http-server --directory $DRM or python -m http.server -d $DRM isn't sufficient due to the fact that none of the hyperlink targets contain file extensions. According to cpage this is somehow "correct", but it sure isn't useful.

An alternative is to do what is suggested in #2 and convert the DRM to Sphinx. I quite favor that, actually, but it's a lot more work.

In any case, here's the necessary nginx config (I believe) from opendylan.org:

        location /books/drm/ {
                index Index.html;
                try_files $uri $uri/ $uri/Index.html $uri.html $uri.png $uri.jpg =404;
        }