Clarify interaction of Cargo.toml and pyproject.toml metadata

[emmatyping]

I was reading through the readme about PEP 621 support in the Python metadata section and I realized that it isn't clear how it interacts with metadata listed in Cargo.toml. Which values are sourced from where and what if they disagree?

It appears that they are merged, but it would be nice to have this written down somewhere so it is clear.

EDIT: it seems that these values are merged, and that pyproject.toml takes precedence

[messense]

pyproject.toml take precedence over Cargo.toml. #465 (comment)

I'll update README.md for this later.

[emmatyping]

Thank you!