Document ipynb formatting for Exercises and Challenges

The scripts for converting from ipynb to html successfully depend on a specific formatting for Exercises and Challenges. In particular, the script that builds the [all exercises and challenges](http://introtopython.org/all_exercises_challenges.html) page needs specific formatting.

This should probably be documented in the readme, or in a page linked to from readme. Research the conventions for supporting readme pages; more markdown pages, or make a wiki? I lean towards markdown, but not sure if that's conventional. I like markdown pages because they are part of the repo that gets cloned.

Exercise sections should follow this format:
- Make a new cell.
- Make a line with the label "Exercises", followed by ---
- Make a series of exercises, with titles preceded by ####
- Start a new cell, with a link to top

This is what the code in your notebook should look like:

```
Exercises

---
#### Title of Exercise
- specific directive
- more specific directive

#### Title of Exercise
- specific directive
- more specific directive

[new cell]
[top](#)
```

The script for all_exercises_challenges looks for the `top` cell when scraping for exercises and challenges.

Sets of Challenges follow the same format, with the word 'Challenges' in place of 'Exercises'.