docs: add Playground url (Docx.js Editor) to README.md

[naskio]

Hi @dolanmiu
I recently created Docx.js Editor, a browser-based editor that provides live preview while coding with docx.js.
Adding this resource to the README would help users experiment with the library and visualize results in real-time.
Let me know if you need more details/context.

[dolanmiu]

This is very cool. I am very impressed!

Would be great if there are are complex/complete examples

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4e2befb) to head (96f5123).
Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files          287       287           
  Lines         8387      8387           
  Branches      1410      1410           
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  Hits          8387      8387           

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[naskio]

Thank you for merging this PR
You're right - having more complex examples would be valuable.
If you'd like to contribute, you can add more templates directly to the /public/templates folder (Just create a new file there)