readme: add security policy (#649) #507

Job	Run time
miri	8m 23s
loom	1m 37s
minrust	1m 40s
cross (i686-unknown-linux-gnu)	1m 6s
cross (armv7-unknown-linux-gnueabihf)	1m 12s
nightly	2m 44s
rustfmt	10s
cross (powerpc-unknown-linux-gnu)	1m 11s
stable (ubuntu-latest)	1m 17s
cross (powerpc64-unknown-linux-gnu)	1m 11s
stable (macos-latest)	2m 7s
tsan	2m 5s
cross (wasm32-unknown-unknown)	14s
stable (windows-latest)	2m 12s
Publish Documentation	1m 27s
	28m 36s

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