docs: add Tutorial section to README

[Oval17]

Summary

• Adds a step-by-step Tutorial section to Readme.md, placed between the existing "How It Works" and "Build" sections.
• The tutorial is structured as 7 numbered subsections, each explaining a distinct concept or pipeline stage in cshell.c.
• Every code snippet is taken verbatim from the actual source — no invented pseudo-code.

Subsections added

1. Constants and the Trim Helper
2. The Read-Eval Loop
3. Built-in Commands (cd, exit)
4. Splitting the Command Line into Pipeline Stages
5. Forking and Wiring the Pipe Chain
6. Parsing a Single Segment — Arguments and Redirection
7. Applying Redirection and Executing

Test plan

• Review that all code snippets match the source in cshell.c exactly.
• Confirm section ordering: How It Works → Tutorial → Build → Run → Controls.
• Read through each subsection for accuracy and clarity.

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