Expand docs for HTTPS CA validation
Expand docs for HTTPS CA validation

Informs administrator where they can make edits on the app server
in order to prove to a CA that they own an onion domain.

heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop