docs: add Tutorial section to README

[Oval17]

Summary

• Adds a step-by-step Tutorial section to Readme.md between the existing "How It Works" section and the "Build" section.
• The tutorial is split into 6 numbered subsections covering: simulation constants & state arrays, the driven anchor node, Verlet integration with damping and gravity, distance constraint solving, rasterising nodes into a character buffer, and frame rendering & timing.
• Every code snippet is taken verbatim from rope.c — no invented pseudo-code.

Test plan

• Read through each subsection and verify every code snippet matches the corresponding lines in rope.c.
• Build and run the program (gcc rope.c -o rope -lm && ./rope) to confirm the described behaviour matches the tutorial text.
• Check that the rest of the README is unchanged.

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