readme: some improvements (#1358) #2411

Job	Run time
mac (stable, x86_64-apple-darwin)	16m 48s
mac (nightly, x86_64-apple-darwin)	9m 22s
windows (stable, x86_64-pc-windows-gnu)	9m 58s
windows (stable, x86_64-pc-windows-msvc)	8m 28s
windows (nightly, x86_64-pc-windows-gnu)	5m 0s
windows (nightly, x86_64-pc-windows-msvc)	6m 54s
linux (stable, i686-unknown-linux-gnu)	8m 22s
linux (stable, i686-unknown-linux-musl)	8m 18s
linux (stable, x86_64-unknown-linux-gnu)	8m 49s
linux (stable, x86_64-unknown-linux-musl)	9m 44s
linux (stable, aarch64-unknown-linux-gnu)	7m 17s
linux (beta, i686-unknown-linux-gnu)	5m 31s
linux (beta, i686-unknown-linux-musl)	7m 30s
linux (beta, x86_64-unknown-linux-gnu)	5m 31s
linux (beta, x86_64-unknown-linux-musl)	5m 39s
linux (beta, aarch64-unknown-linux-gnu)	5m 37s
linux (nightly, i686-unknown-linux-gnu)	6m 16s
linux (nightly, i686-unknown-linux-musl)	7m 28s
linux (nightly, x86_64-unknown-linux-gnu)	5m 55s
linux (nightly, x86_64-unknown-linux-musl)	6m 57s
linux (nightly, aarch64-unknown-linux-gnu)	5m 56s
	2h 41m 20s

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