Include Notebook Difficulty Level in README

[kadrlica]

During the PCW it became clear that users had no way of knowing how advanced a notebook would be before starting it. The idea would be to assign a category: novice, intermediate, expert (or green circle, blue square, black diamond?) to each notebook. This could be stored in the notebook header and on the readme. We'd probably need feedback from users at various levels to accurately categorize notebooks. @Madejski @drphilmarshall ?