User guide

[Notgnoshi]

The README will be too large an overwhelming if it documents everything, so put together a docs/getting-started.md document, that should cover:

1. Installation
2. Application configuration
   1. Where to find
   2. Rule settings
3. Adding repositories
   1. Auth
   2. Re-using existing checkouts
4. Running Herostratus regularly (cron / systemd timer)
5. Integrations (future)