Add README with speed comparison + cosine emulation docs #151

Job	Run time
format/stable	18s
clippy/stable	22s
cross_test/i686-unknown-linux-gnu/stable	1m 32s
Pass MSRV values to other jobs	3s
cross_test/s390x-unknown-linux-gnu/stable	1m 41s
nostd/thumbv6m-none-eabi/stable	28s
native-backend/stable	40s
docs/nightly	0s
miri	0s
cargo-careful	0s
tests/1.64.0	1m 1s
tests/beta	54s
blas-msrv	58s
tests/stable	47s
conclusion	0s
	8m 44s

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