Add AFL coverage instructions #2941

Job	Run time
test (1.81)	1m 40s
test-llvm (14, ubuntu-22.04)	1m 48s
test (stable)	1m 44s
test-llvm (15, ubuntu-22.04)	1m 47s
test (beta)	1m 42s
test-llvm (16)	1m 50s
test (nightly)	2m 37s
test-llvm (17)	1m 48s
test (nightly, macos-latest)	2m 29s
test-llvm (18)	53s
test (nightly, windows-latest)	4m 33s
test-llvm (19)	2m 32s
miri / miri	2m 24s
msrv / prepare	2s
release-dry-run / prepare	16s
tidy / prepare	2s
tidy / clippy	20s
tidy / check-external-types	0s
tidy / tidy	56s
tidy / docs	0s
tidy / deny / deny	17s
msrv / build (ubuntu-latest)	3m 12s
msrv / build (windows-latest)	3m 10s
release-dry-run / build (aarch64-unknown-linux-gnu, ubuntu-20.04)	1m 28s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 23s
release-dry-run / build (aarch64-apple-darwin, macos-13)	2m 7s
release-dry-run / build (x86_64-unknown-linux-gnu, ubuntu-20.04)	1m 6s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 14s
release-dry-run / build (x86_64-apple-darwin, macos-13)	1m 57s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 50s
release-dry-run / build (x86_64-unknown-freebsd)	1m 29s
release-dry-run / build (universal-apple-darwin, macos-13)	3m 14s
	53m 50s

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