docs: add sponsors section to README

[maxktz]

Summary

• add a Sponsors section to the README
• embed the live GitHub Sponsors grid with dark and light variants

Validation

• git diff --check
• verified the GitHub Sponsors page and sponsor SVG endpoint resolve successfully

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Summary by cubic

Adds a Sponsors section to the README that embeds the live GitHub Sponsors grid for PayKit. The grid adapts to the viewer's color scheme, showing dark or light variants automatically.

^{Written for commit fdc4008. Summary will update on new commits.}

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Project	Deployment	Actions	Updated
paykit	Ready	Preview	Sep 18, 2026 11:51am UTC

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[maxktz]

Superseded by #211, which uses the existing feat/ui-fixes branch.