Document the relevant security aspects of the Tails Verification / Installation Assistant

Originally created by @sajolida on #15061 (Redmine)

https://mailman.boum.org/pipermail/tails-dev/2017-November/011916.html

« Once we’re done here, I’d like to see the security reasoning that
leads to the “it’s safe” conclusion documented somewhere in the
extension Git repo: we want our code to be friendly to auditors, and
mailing list archive are not an obvious place to find such doc.
@sajolida: I volunteer to take care of this if Uzair does not. »

Feature Branch: contrib/15061-document-security-of-verification-extension

Parent Task: #12328 (closed)

Related issues

• Related to #14815 (closed)