cpufeatures: use `#![doc = include_str!("../README.md")]` (#1193) #1849

Job	Run time
test (stable)	9s
build (stable, wasm32-unknown-unknown)	19s
build (1.85.0, wasm32-unknown-unknown)	16s
build (1.85.0, thumbv7em-none-eabi)	18s
minimal-versions / minimal-versions	29s
test (1.85.0)	14s
build (stable, thumbv7em-none-eabi)	17s
	2m 2s

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