docs: add a smile to the README welcome

[zimeg]

Summary

• Add 😄 to the README welcome message, matching the existing emoji style.
• Keep this change scoped to README.md.

Validation

• git diff --check passed before committing.
• Runtime tests not run (documentation-only change).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.20%. Comparing base (a9bda96) to head (f1c1507).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #694      +/-   ##
==========================================
- Coverage   78.21%   78.20%   -0.01%     
==========================================
  Files         239      239              
  Lines       18127    18127              
==========================================
- Hits        14178    14177       -1     
- Misses       3949     3950       +1     

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[zimeg]

🎣 I smile closing this PR. Thanks for reading this message.