Add Security guide

[florimondmanca]

Is your feature request related to a problem? Please describe.
Security is an important topic, and especially as Bocadillo is a young framework, we need to address it.

Describe the solution you'd like
Add a "Security" topic guide discussing common gotchas of web security (MITM, XSS, CSRF, SQL injection…) and how to protect against those.

Additional context
Motivated by #12