Clarify inclusion of 3rd party resources in design document

    our generated files in a different directory, the paths won't work

    there).

7.  In order to display properly, our generated HTML pages need

    artwork, CSS style files, and a few bits of Javascript.  We could

    load these from the web, but that would make offline authoring

    difficult.  Instead, each lesson's repository has a copy of these

    files, and a way of updating them (and only them) on demand.

7.  In order to display properly, our generated HTML pages need artwork,

    CSS style files, and a few bits of Javascript.  We could load these

    from the web e.g. from a content delivery network (CDN), but that

    would make offline authoring difficult.  Instead, each lesson's

    repository is self-contained and has a copy of all these third party

    resources, and a way of updating them (and only them) on demand.

One final note: we try not to put HTML inside Markdown because it's

ugly to read and write, and error-prone to process. Instead, we put