README.md: update `rfc6979` description #1472

Job	Run time
test (1.72.0)	21s
build (thumbv7em-none-eabi, 1.72.0)	19s
test (stable)	10s
build (thumbv7em-none-eabi, stable)	16s
build (wasm32-unknown-unknown, 1.72.0)	17s
build (wasm32-unknown-unknown, stable)	22s
	1m 45s

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