Add GitHub Sponsors badge to README #55

Job	Run time
test-macos	5m 33s
cross-compile (aarch64-linux-gnu)	3m 46s
test-linux	3m 49s
cross-compile (x86_64-macos-none)	3m 41s
cross-compile (aarch64-macos-none)	4m 5s
cross-compile (x86_64-linux-gnu)	3m 43s
	24m 37s

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