Update section about PGP

[runasand]

While PGP can be used to send encrypted emails, it can also be used to verify digital signatures for software (e.g. the Tor Browser Bundle). It might make sense to talk about some of the pitfalls when trusting the Thunderbird extension and talk about other ways to use PGP (e.g. copy/paste between browser and text editor). Perhaps also mention how one can export and import keys, as well as upload/download keys using a key server. I hear this section also has some missing steps.