Added a basic overview of the SecureDrop system.
Added a basic overview of the SecureDrop system.

Currently, there's no one place to go if you already know that you're
interested in SecureDrop and want to know what the major pieces are and
how everything fits together. This overview document is intended to fill
that gap.

conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,ageis/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ageis/securedrop,pwplus/securedrop,micahflee/securedrop,ageis/securedrop,pwplus/securedrop,micahflee/securedrop,pwplus/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,pwplus/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,ageis/securedrop,garrettr/securedrop,garrettr/securedrop,pwplus/securedrop,conorsch/securedrop,ehartsuyker/securedrop,pwplus/securedrop,heartsucker/securedrop