Add AFL coverage instructions #2870

Job	Run time
test (stable)	1m 55s
test-llvm (14)	1m 36s
test (beta)	1m 57s
test-llvm (15)	1m 27s
test (nightly)	2m 36s
test-llvm (16)	1m 43s
test (nightly, macos-latest)	2m 29s
test-llvm (17)	2m 8s
test (nightly, windows-latest)	4m 18s
test-llvm (18)	1m 9s
test-llvm (19)	1m 31s
deny / deny	14s
miri / miri	2m 21s
msrv / msrv	5m 21s
tidy / tidy	1m 10s
release-dry-run / prepare	5s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 25s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 24s
release-dry-run / build (aarch64-apple-darwin, macos-13)	3m 30s
release-dry-run / build (x86_64-unknown-linux-gnu)	1m 14s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 11s
release-dry-run / build (x86_64-apple-darwin, macos-13)	2m 50s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	2m 13s
release-dry-run / build (x86_64-unknown-freebsd)	1m 52s
release-dry-run / build (universal-apple-darwin, macos-13)	5m 16s
	52m 55s

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