Rewrite the README as a user guide

The README is a guide for the user: setup of the data directory, login,
Telegram, WhatsApp, the user service, and backups, then the client
commands, what the agent does, the log, and development. It drops the
internal files, formats, locks, queues, recovery details, and limits
that each feature had added, which the doc comments of their code hold.
It shrinks from 703 to 240 lines.