docs: add FOSS United to acknowledgements

[SuperCoolPencil]

Added FOSS United to the acknowledgements section for providing a grant and their admirable work in growing the FOSS ecosystem in India.

Greptile Summary

This PR adds FOSS United to the acknowledgements section of README.md, crediting them for a development grant and their work growing the FOSS ecosystem in India.

• A single line is inserted before the existing Charm and GoReleaser Pro entries, following the same markdown list format with a hyperlink and brief description.

Confidence Score: 5/5

Purely a documentation addition with no code changes; safe to merge as-is.

The change is a single-line addition to the acknowledgements list in README.md. The link format is consistent with existing entries and the URL points to the correct organization. No functional, logic, or security concerns are introduced.

No files require special attention.

Important Files Changed

Filename	Overview
README.md	Adds FOSS United to the acknowledgements section with a link and description of their grant; documentation-only change with no functional impact.

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