README: rand is not a crypto library #1089

Job	Run time
Check doc	31s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 9s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	2m 0s
test (macos-latest, x86_64-apple-darwin, stable)	1m 3s
test-ios	21s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 52s
test-miri	4m 26s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 55s
test-no-std	13s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 36s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	2m 13s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 46s
	21m 5s

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