SemVer policy + MSRV

[tarcieri]

Coming from RustCrypto/password-hashes#627

I think it would make sense to replace the old MSRV sections of our README.md with a new "SemVer Policy" section which lists MSRV as being out-of-scope, possibly highlighting rust-lang/api-guidelines#231

Without it, I worry people will say MSRV changes are a breaking change (even if that's wrong), so it would be nice to have something to point to in the README saying MSRV changes don't violate our SemVer policy.

[newpavlov]

Sounds good to me. Something like this?

Minimum Supported Rust Version (MSRV) Policy

The crate MSRV accounts for all supported targets and crate feature combinations, excluding explicitly unstable features.

MSRV increases are not considered breaking changes and can happen in patch releases.

[tarcieri]

Sure, although maybe reverse the order of those two lines?

MSRV increases are not considered breaking changes and can happen in patch releases.

The crate MSRV accounts for all supported targets and crate feature combinations, excluding explicitly unstable features.