Add dev guide link to readme

[jeffpaul]

What?

Closes #225.

After merging #501 and #475, the final bits to close out #225 are this bullet addition to the readme.

Copilot summary

This pull request updates the documentation to provide more guidance for plugin developers. The main change is the addition of a link to the new Developer Guide in the README.md.

Documentation improvements:

• Added a link to the new Developer Guide (docs/DEVELOPER_GUIDE.md) in the README.md to help developers contribute to the plugin or create their own AI-powered experiments.

Why?

How?

Use of AI Tools

None, all slow docs review by my brain.

Testing Instructions

Screenshots or screencast

Before	After

Changelog Entry

Changed - Documentation updates.

[github-actions]

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Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.21%. Comparing base (d8c6c92) to head (e3f02b9).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #569   +/-   ##
==========================================
  Coverage      71.21%   71.21%           
  Complexity      1151     1151           
==========================================
  Files             67       67           
  Lines           5565     5565           
==========================================
  Hits            3963     3963           
  Misses          1602     1602           

Flag	Coverage Δ
unit	71.21% <ø> (ø)

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