Explain cost function in README. #878

Job	Run time
Rust format	17s
Rust check	23s
Rust doc	1m 28s
Rust check with default features	23s
Rust test	2m 6s
Rust check (windows-latest)	50s
Rust check (macos-latest)	1m 0s
Rust lint (beta)	22s
Rust lint	21s
Rust lint (nightly)	21s
Rust lint (stable)	19s
	7m 50s

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