added a small note to illustrate why the shell is called the shell, ther...

...e would be plenty of illustration available for this particular metaphor (think http://en.wikipedia.org/wiki/The_Birth_of_Venus_(Botticelli)), but that is to the speaker to decide upon

I hereby like to apply to become a swc instructor

PR motivated by https://github.com/swcarpentry/bc/pull/820#issuecomment-60942227