Add simplified contribution model diagram for educational clarity

[longnp667464-lgtm]

Summary

This pull request adds a simplified contribution model diagram to the README
to help readers better understand how contributors collaborate asynchronously
using GitHub’s fork-and-pull-request workflow.

What was added

• A clear ASCII-based contribution model diagram.
• A short explanation describing asynchronous collaboration and maintainer review.
• Educational context linking the contribution process to distributed systems concepts.

Why this change is useful

Although this repository is documentation-focused, the contribution workflow
itself is an excellent real-world example of:

• Distributed version control
• Asynchronous collaboration
• Event-driven workflows (Pull Requests, reviews, CI)

This addition improves clarity for students, educators, and newcomers learning
about open-source collaboration models.

Scope of change

• Documentation-only change
• No impact on site functionality or build process

Thank you for maintaining Open Source Guides!