docs(readme): add architecture diagram

[cfm]

Status

Ready for review

Description

@deeplow and I have walked through the log/ portion of this repository and mapped out:

• what spawns what services;
• what triggers what RPCs; and
• what reads/writes from/to what.

It should be descriptive and uncontroversial, except that we couldn't account for:

1. securedrop-log, which nothing in either securedrop-client or securedrop-workstation appears to invoke; or
2. securedrop_log, which nothing in either securedrop-client or securedrop-workstation appears to load.

Are they remnants of an earlier implementation, before the securedrop-redis-log → securedrop-log-saver buffering pipeline and the sd-rsyslog → qrexec securedrop.Log RPC?