Update contribution guidelines to use --index

[Deedasmi]

Introduced in rust-lang/cargo#4209

[kureuil]

Thanks for the pull request @Deedasmi :)

If I remember correctly cargo isn't automatically updated, so wouldn't it be safer to keep the old flag in the documentation, and specify the version ranges compatible with each flag (of course, we'd make --index the default) ?

What happens when you run cargo publish --host file:///path/to/your/crates.io/checkout/tmp/index-co on the latest cargo version ?

[Deedasmi]

A friendly deprecation warning saying --host is no longer valid, and that it will 'soon' become a hard error.

I do believe rustup update automatically updates cargo, so most should be getting updated sooner rather than later. I'm cool with adding a footnote or just closing this and revisiting it later, after a couple more updates.

[kureuil]

I think leaving a footnote for old versions of cargo is enough for now 👍

[carols10cents]

Love it!!! Thank you!!

bors: r+

[bors-voyager]

Merge conflict (retrying...)

[bors-voyager]

Build succeeded

• continuous-integration/travis-ci/push