docs: add Tutorial section to README

[Oval17]

Summary

• Adds a Tutorial section to Readme.md, placed between the existing "How It Works" section and the "Build" section.
• The tutorial contains 7 numbered subsections covering every major concept in relay.c: the Client struct and slot array, socket setup, the select() event loop, accepting connections, broadcasting messages, the /nick command, and the live dashboard.
• All code snippets are quoted verbatim from relay.c — no invented pseudo-code.

Test plan

• Verify the rendered Readme on GitHub looks correct (headings, code blocks, ordering).
• Confirm the Tutorial section appears after "How It Works" and before "Build".
• Check that every code snippet matches the corresponding lines in relay.c.

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