Update chapter07.rst

For the example to work, you have to use {% csrf_token %} and additional imports in views.py
Added short documentation about csrf_token

This is the first time csrf_token is used in the book, the short
explanation comes from the django tutorial, part 4, as my english is
kind of weak...
Added link to relevant chapters
Merge pull request #17 from Lapin-Blanc/patch-1

Update chapter07.rst