Add brief inventory of PoC implementation

[eloquence]

In reading through this codebase, I stumbled a bit over the combination of server, journalist client and source client in the same repo. My understanding is, broadly:

• Redis is used as the server's data store
• SQLite is used for the journalist client DB, expected to hold decrypted messages
• keys/ directory is used for the journalist client PKI
• files/ directory is used for uploaded server files (and downloaded journalist files?) and journalist client DB
• commons.py includes functions that are used by both journalist and source clients

If that's broadly correct, I think a similar brief summary would be nice to have in the README -- and to make it explicit that there is no full separation of concerns in the PoC (by which I mean, for example, that files both for the server and the journalist client are stored in the same directory).

[eloquence]

(Happy to take a swing at this, just wanted to check my understanding first & see if other folks would find this useful.)