Update chapter07.rst

[Pythnist]

updating ch-07 b adding csrf token in post method

[jacobian]

Probably can't just drop this in here without a brief note about it. Is this the first place csrf_token is used? If so we should add a note about the fact that's required for all forms and pointing readers to the security chapter (20?) for more details.

[jacobian]

Fixed in #17.