readme: bump version to 0.6.0 #255

Job	Run time
cargo fmt --all -- --check	5s
cargo clippy -- -D warnings (ubuntu-latest)	9s
cargo check (ubuntu-latest)	8s
Code Coverage (ubuntu-latest, unix)	1m 40s
cargo clippy -- -D warnings (macOS-latest)	15s
cargo check (macOS-latest)	11s
Run the fuzzers	2m 32s
cargo clippy -- -D warnings (windows-latest)	33s
Code Coverage (macos-latest, macos)	2m 25s
cargo check (windows-latest)	1m 18s
Code Coverage (windows-latest, windows)	3m 40s
cargo test (ubuntu-latest)	18s
cargo test (macOS-latest)	13s
cargo test (windows-latest)	1m 39s
	15m 6s

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