[Migrated] Document resolver = "2" effective requirement for rust-gpu

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/1022
Old labels: t: bug
Originally creatd by Firestar99 on 2023-04-03T10:47:39Z

Add official documentation that one should have

[workspace]
resolver = "2"

in their root cargo.toml to make sure dependencies are resolved correctly and that crates compiled with different features on native and spirv don't get merged into one feature set.

An example can be num-traits which compiles with features default,std on native and libm on spirv target.

For a practical example see the issue below:
https://github.com/EmbarkStudios/rust-gpu/issues/986

issue 986 の実践的な例を読み、その後、プロジェクトのドキュメントとルートの Cargo.toml にあるガイダンスを確認してください。[workspace] resolver = "2" を示し、native と SPIR-V の feature set がマージされるのを防ぐことを説明する公式ガイダンスを追加してください。resolver の要件と num-traits の例がドキュメント化されていれば完了です。