Add instructions for installing with cargo #6

Job	Run time
install-cross (linux-musl)	5s
macos (x86_64-apple-darwin)	1m 28s
macos (aarch64-apple-darwin)	1m 27s
windows (i686-pc-windows-msvc)	2m 6s
linux (aarch64-unknown-linux-gnu)	1m 9s
windows (x86_64-pc-windows-msvc)	2m 18s
linux (aarch64-unknown-linux-musl)	1m 17s
linux (arm-unknown-linux-gnueabi)	1m 15s
linux (arm-unknown-linux-gnueabihf)	2m 18s
linux (armv7-unknown-linux-gnueabihf)	1m 7s
linux (i686-unknown-linux-musl)	2m 13s
linux (powerpc64le-unknown-linux-gnu)	1m 25s
linux (x86_64-unknown-linux-musl)	1m 20s
	19m 28s

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