MSRV Policy wording

[alvierahman90]

README.md states:

MSRV bumps are considered breaking changes and will be performed only with minor version bump.

Should this state MSRV bumps are not considered breaking, or that bumps will only be done with major version bump?

[newpavlov]

All crates in this repository are pre-v1.0, so minor version bumps effectively signal breaking changes.

Note that this policy will be relaxed in the upcoming breaking releases and we will allow MSRV bumps in patch releases thanks to the MSRV-aware resolver.

[tarcieri]

FWIW I think we should retain some wording about MSRV in our READMEs, even just to mention that we don't consider MSRV breaking as part of the SemVer policy (possibly linking rust-lang/api-guidelines#231)