Add AFL coverage instructions #2938

Job	Run time
test (1.81)	1m 43s
test-llvm (14, ubuntu-22.04)	1m 49s
test (stable)	1m 47s
test (beta)	1m 41s
test-llvm (15, ubuntu-22.04)	1m 47s
test (nightly)	2m 29s
test-llvm (16)	1m 54s
test (nightly, macos-latest)	3m 29s
test-llvm (17)	1m 49s
test (nightly, windows-latest)	5m 17s
test-llvm (18)	50s
test-llvm (19)	1m 20s
miri / miri	2m 22s
msrv / prepare	6s
release-dry-run / prepare	3s
tidy / prepare	3s
release-dry-run / build (aarch64-unknown-linux-gnu, ubuntu-20.04)	1m 31s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 21s
release-dry-run / build (aarch64-apple-darwin, macos-13)	3m 13s
release-dry-run / build (x86_64-unknown-linux-gnu, ubuntu-20.04)	1m 3s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 20s
release-dry-run / build (x86_64-apple-darwin, macos-13)	1m 52s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 41s
release-dry-run / build (x86_64-unknown-freebsd)	1m 51s
release-dry-run / build (universal-apple-darwin, macos-13)	2m 44s
tidy / clippy	20s
tidy / check-external-types	0s
tidy / tidy	57s
tidy / docs	0s
tidy / deny / deny	16s
msrv / build (ubuntu-latest)	2m 22s
msrv / build (windows-latest)	3m 16s
	54m 16s

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