cpufeatures: add mentions of `avx512vbmi` and `avx512vbmi2` to readme… #1463

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	2m 21s
build (1.56.0, wasm32-unknown-unknown)	2m 59s
build (stable, thumbv7em-none-eabi)	22s
build (stable, wasm32-unknown-unknown)	24s
test (1.56.0)	2m 53s
test (stable)	21s
minimal-versions / minimal-versions	26s
	9m 46s

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