Update README with quick‑start guide, Dockerfile, and example screenshots

[emperor42]

Add a “Getting Started” section that shows go run stenella.go and how to open the UI.
Provide a minimal Dockerfile that builds the binary and runs it (EXPOSE 8080).
Include a screenshot of the UI (feed list + source manager).
Document all configuration knobs (max sources, cache TTL, etc.) and how to enable extra handlers.