IPython docs

**Instructions for Review**

1. Use the **Files changed** view (or view a specific commit).
2. For the notebook you want to review, click the "View" button near the file name.
3. Copy the URL you're now viewing.
4. Open http://nbviewer.ipython.org/
5. Paste the URL in the text box
6. Leave feedback in-line if you can find what you're looking for in the JSON, or just as a comment on the Pull Request with a link to the notebook you're viewing.