Add removed paragraphs to Architecture.md

`arti`:  A simple command-line client program that can run as a SOCKS proxy.

## Design considerations, privacy considerations.

As we build the APIs for Arti, we've been aiming for

simplicity and safety: we want it to be as easy as possible to use

`arti-client`, while trying to make certain kinds of privacy or security

violation hard to write accidentally.

The lower-level we get, however, the more safety we lose.  If we need to

allow a piece of functionality that isn't safe for general purposes, we

usually put it at a more low-level crate.

Privacy isn't just a drop-in feature, however.  There are still

plenty of ways to accidentally leak information, even if you're

anonymizing your connections over Tor.  We'll try to document

those in a user's guide at some point as Arti becomes more mature.