bump Cargo.toml minimum version

bump Cargo.toml minimum version

release notes

add emphasis

Merge pull request rayon-rs#190 from nikomatsakis/release-v0.6.0

release v0.6.0

Merge pull request rayon-rs#134 from nikomatsakis/release-0.5.0

Release Rayon 0.5.0

Merge pull request rayon-rs#121 from nikomatsakis/release-v0.4.3

Release v0.4.3

Merge pull request rayon-rs#99 from nikomatsakis/release-v0.4.1

Release v0.4.1

Release v0.4.1

add build badge